From Confidential Service Memorandum MG/338 dated 22 Nov, 1960:

The aluminum hard top assembly, Part No. AHH5562, is no longer available and a fibre-glass (black Vynide-covered) type, Part No. AHH5991, will be offered in its place.

HARD TOP AND SLIDING SIDESCREENS
To ensure that the correct type of sliding sidescreen is used, refer to the chart below.

Follow up notes and implications:

Note that the chart shows only two basic models of sidescreen, in LH and RH part numbers. At this point they would supplant a wider range of original model sidescreens.

1.)  Early aluminum hard tops were originally supplied with sidescreens with black painted frames. This was shortly changed to bare aluminum frames. Apparently the bare aluminum model supersedes the black model, so any successive replacement parts would be bare aluminum, and would be correct period replacement parts for aluminum hard top. Recall from earlier article that the sidescreens for the aluminum hardtop have rubber seals in the front on an the bottom, but not on top or back.

2.)  Fibre-glass hardtops were supplied with a slightly different form of bare aluminum sidescreen having a straight frame along the bottom rather than tapered, but still no seal along the top or back.

3.)  1500 soft top cars were originally equipped with fabric covered sidescreens with fixed lights and flip-up bottom panel (including early Twin Cam cars). These are apparently superseded by the part number for the aluminum frame model from the aluminum hard top (with front and rear sliding panels), which would be correct period replacements for the 1500 soft top cars. So far I have no indication that the factory ever added a top and rear edge seal (as many modern aftermarket parts have). By the mid 1960's it was very common for 1500 cars to be sporting the bare aluminum frame sidescreens with no top or rear seal,. and the original flip-up type were already exceeding rare on the street. Some aftermarket sidescreens with bare aluminum frames were also available by that time (always slightly different from the factory parts). Apparently the original flip-up sidescreens were commonly shunned (hated?) for their lack of ventilation. If you bought a hardtop for your MGA 1500 roadster prior to introduction of the 1600 model, whether from factory or dealer, you would get the aluminum hardtop, and you needed new sidescreens to match anyway.

4.)  1600 soft top cars were originally equipped with fabric covered sidescreens with fixed Izinglas front light and sliding rear light (including 1600 style Twin Cam cars). These had rubber seals on front and bottom, and a wide fabric flange at top and back. Introduction of the fibre-glass hard top coincides with introduction of the 1600 model car. At that time (or shortly thereafter) the 1600 type sidescreens were made to conform to fit the fibre-glass hardtop and the soft top cars (and were still fabric covered). The factory never made a fabric covered sidescreen with two sliding lights. If you bought a hardtop for your MGA 1600 roadster from the factory, it would be fibre-glass, and the sidescreens would fit both the soft top and the hardtop as originally intended. This would carry over into production of the 1600-MK-II model cars.

5.)  It is believed that aluminum tops were not shipped to the dealers after start of production of the 1600 model (except perhaps on special order), so aluminum hard tops would be replacement type (dealer aftermarket) for the 1500 style roadsters prior to that time, and the fibre-glass hard top would be the correct period accessory for any MGA roadster later (except for a few rare instances of left over stock of aluminum tops). Since the 1500 car would need different (non-original) sidescreens for use with the hard top, it would likely be sporting the bare aluminum sidescreens to match the aluminum hardtop, or the 1600 style sidescreens to match the fibre-glass hard top, depending on when the hard top was purchased.

6.)  Only five Twin Cam roadsters are known to have been shipped from the factory with hardtops (all aluminum hardtops). These were the five 1960 Sebring race cars, and none of them had a soft top because of race rules for space requirement for luggage. For any other Twin Cam roadster, if you bought a hardtop through a dealer (with original purchase or later) the type of hardtop would be whatever the dealer was supplying at the time. This would be aluminum early, or fibre-glass after advent of the 1600 (with rare later exceptions for left over stock of aluminum tops).

7.)  There was a period of 14 months after introduction of the 1600 model (May 1960 to July 1961) when both the aluminum and fibre-glass hard tops were in production at the same time. During this period the factory was fitting aluminum hardtops to many of the MGA roadsters with the Competition Suspension option. These cars later came to be called the "Deluxe" model in either 1600 or 1600-MK-II form. When these cars were equipped with the aluminum hardtop at the factory they would have the matching bare aluminum frame sidescreens. There is no indication that the factory ever supplied two different sets of side curtains with any one car, so if you happen to see a "Deluxe" rag top car sporting bare aluminum sidescreens you might ask the owner if there is a hardtop to go with it (or otherwise assume someone was just frugal when buying replacement sidescreens).

8.)  When stock of aluminum hard tops was depleted, fibre-glass hard tops would be optionally fitted to later Deluxe roadsters, same as for the standard roadsters. As such, some late production 1600 Deluxe cars and any 1600-MK-II Deluxe would use the fibre-glass hardtop for originality.

9.)  Now more than 40 years later there is a lot of discussion among the MGA Twin Cam group about fitting hardtops to many of the Twin Cam roadsters. As the only (5) MGA Twin Cam cars known to leave the factory with hardtops had aluminum tops, most of the current Twin Cam owners seem to much prefer the aluminum hard top, as being the correct period accessory at the time the cars were originally sold. However there is no shame in having a fibre-glass hard top on your MGA Twin Cam roadster, as this is the correct dealer supplied accessory being sold after the MGA 1600 type body style was introduced. As always, the aluminum hard top should be matched with complimentary bare aluminum sidescreens (or black for very early ones), and the fibre-glass hardtop should be matched with the complimentary 1600 style bare aluminum sidescreens.

10.)  In August 1960, several months before introduction of the 1600-MK-II model, the soft top was changed (for better weatherproofing) to include an inward step in the fabric construction around the side openings. The sidescreens were also modified to remove the rear flap for better fit with the new top configuration. Note at the top of this page the date of the Confidential Service Memorandum is a few months later. This indicates that the sidescreens for the 1600 model, as referred to in this memo, have been superseded to the latest version with the rear flap removed. So now any fabric covered sidescreens (the 1600 type) purchased as replacements would be correct period accessories without the rear flap.

11.)  The aluminum framed sidescreens (match for the aluminum hard top) were considerably less expensive than the fabric covered parts. Replacement part sales of the aluminum framed parts could have continued to be popular for as long as they were available. Indeed to this day the less expensive bare aluminum frame sidescreens are still in demand as replacement parts (all being aftermarket supply for recent decades). When it comes to originality, it is hard to tell anyone that their MGA sidescreens may not be original, or at least correct period accessories, regardless of what type of sidescreen may be used. To do that you have to narrowly define the term "period".

I was just getting used to the idea of what the meaning of the word "is" is, and now we have to define period?