ARB	AUTO	BAX	BEES
DORMAN	FCF	GKN	LINREAD
NEWTON	ROCKFORD	RUBERY OWEN	SPARTS
TWL	WILEY R	WILEY S	WODEM

MGA 1500 engine BP15GB-17650 ( about Mar 1957)
original oil pan screws are LINREAD brand

MGA 1600 MK II, car number GHNL2 /108789 (May 1962)
original oil pan screws are NEWTON brand.

When I pulled 4 screws 5/8" long out of my old inventory (lots of used screws) I found
2 were LINREAD, 1 was NEWTON, and 1 was RUBERY OWEN

That RO screw raised my curiosity, so I crawled under my car to look at the 1600 engine and found:

MGA 1600 engine 16GA-U 15806 (about Feb 1959)
original oil pan screws are NEWTON brand.

Not knowing where the RO came from, more curiosity led me to look again in my old inventory. The above picture was the result. These are all 1/4-28 hex head screws of various lengths (but no other diameter). The one thing these screws all have in common (except for the ROCKFORD brand) is the round depression in the center of the head. That is a standard marking feature used to designate the Unified screw thread standard. This thread standard is common to England, Canada and the United States, and would have spread to other parts of the world as such assemblies were passed around.

The ARB screw is new from Moss Motors in August 2004. The ROCKFORD screws I have been buying recently at the local NAPA auto parts store. I'm pretty sure the other 14 came off of my MGA (or some used replacement parts) over the past 27 years or so. It looks like the MG factory was not very particular about where they bought their screws or how often they changed sources.

So for concours purposes you should not have points deducted for the manufacture name on the screw heard, unless some enterprising person can accumulate a data list showing which screws may have been used for which assemblies during specific production periods (good luck there). However it would be generally expected that all screws of one size fastening a single part would be from a single manufacturer (but still not absolutely certain), and that correct vintage screws would have the Unified thread standard circle on the head (with some rare exceptions).

Time for an ale?

Addendum
On 10/13/2020, Mark Wellard wrote:
"I have taken the sump off the low mileage twin cam engine (never dismantled, as far as I can tell) and found that there was a mixture of brands, all the same size. I found Linread, Wiley, Newton, BEES and SPARTS. It's as though someone thought it would be fun to have a few of everything! -- Regards", -- Mark