MGA 1500 used a black gasket for the tail light plinth to body joint.
MGA 1600 used a silver gasket for the tail light plinth to body joint.

The first 1001 MGA 1600-MK-II had silver rubber plinth gasket, same part number both sides, not handed. That changed to Cream color Prestik for the rest of production. There is a note in the SPL saying if the gasket is not available use the Prestik.

AHH6116 Gasket- plinth (rubber) (2) -- Com(c)100352 Fin(c)101352
AHH6116 not available; use 47H9651
47H9651 Prestik - 1/2 x 1/8" (Cream) 60" -- Com(c)101353

Since the gasket was deleted after only 1001 cars, there is a strong implication that the gasket may never have been available as a replacement part. By now all of the original 1001 cars (what still may exist) should have been changed to Prestik (the first time a new gasket was needed). It is likely that no one will ever reproduce the part that may have a market of only a few hundred remaining cars (like a few pieces per year).

Prestik is a peel and stick firm putty type material in sheet.ribbon form. Peel and stick it to the edge of the plinth, install the plinth, then trim away the excess material. This leaves a sealed joint with nearly invisible gasket. The stuff is disposable, use new material for each assembly. It is still available under the same brand name, but you might have to settle for slightly different size of the raw material. After all, it is really just putty in ribbon form.