There are some other things to check if you were concerned about matching original numbers and parts. There is a body number on a stamped steel tag attached (should be spot welded) to the vertical firewall near the right side bonnet hinge. This does not match the Car Number on the large ID plate, and it is of little concern to anyone except a die hard concours fan, and there are precious few of those in the world. It is not uncommon for a car body to have found its way from one chassis to another during some prior repair or restoration work.

From Robin Barker on 27 October 2002
NAMGAR produces a "Worldwide MGA Database" which lists over 11,000 MGAs known to 12 different MGA groups around the world. The 2nd and latest edition of this database was published in June 2000 and documents the chassis #, engine # and body # reported to these groups by their owners. Records range from cars still with their factory original engine and body all the way to cars pieced together from 2 or more parts/donor cars. With so many cars in the database it is easy to pick out the original numbering sequences used by MG for both engines and bodies. The many cars with transplanted engines stand out as do the few with non-original bodies.
It appears that MG started numbering bodies at 20000 for the first MGA 10101 in 1955 and just kept going until reaching body # 79000 at about MGA 66600. Remember that there were a few Twin Cams in there as well which used body #s in the same range so the number of 1500s and Twin Cams built seems to about equal the number of 1500 style bodies built.
For some unknown reason, after MGA 66600 the body #s seem to have started at 1. The owner of MGA 66883 reports engine # 15GDUH5727 and body # 205. The owner of MGA 67176 reports engine # 15GDUH5871 and body # 473. There are many other reports of low body numbers like this. The highest is from MGA 68833 with engine # 15GDUH7540 and body # 1941.
With the introduction of the MGA 1600 at 68851, both the engine and body numbers change to 1600 type. The earliest report is of MGA 69047 with engine # 16GAU259 and body # B203.
The conclusion is that your body and engine are as installed at the factory. MG simply used low body numbers for the last 2000 or so MGA 1500s. Possibly the original plan was for the 1600 to reach production a little earlier. When this did not happen MG may have simply ordered 2000 more bodies and kept building the 1500.

Addendum April 2011:
There must be lots more MGA in the database by now (11 years later), But I don't have access to that data. However, I now have a report of a very early Australian built MGA 1600.

On April 19, 2011, Tim Sanford in Australia wrote:
"My car was assembled at Zetland (suburb of Sydney) by Nuffield. For many years I have known that it is a very early car in the 1600 assembly and its engine number is 16GAU201, which I was told many years ago was the first of the engine number series for MGAs in Australia. When BMC closed the factory, as you no doubt already know, they destroyed all records :-(
My car has the following numbers on the Nuffield ID plate:
Type:     YGHN 2
Car No:   7/ 68906   507
Colour:   1/MAMBA BLACK / 1
The engine number stamped into the block is: 16GAU201
and the body number on the scuttle is B26".

Note: The first engine number in 1600 produciton was 16GAU101. Tim's engine 16GAU201 is 100 units later, but may well be the lowest 1600 engine number shipped to Australia for the CKD cars. Also chassis frames, engines and bodies were mixed up some prior to final assembly. Tim's Car No. ending in 507 would be the sequence number for Australian CKD assembly order.