MGA 1500 and Twin Cam
6K499 Plug - Oil - drain and filler - (2 in differential)
6K499 Plug - Oil - drain - (1 in gearbox, early and mid 1500 and Twin Cam, and some "Deluxe" with the Magnette or Twin Cam type gearbox)

MGA 1600
6K499 Plug - Oil - drain and filler - (2 in differential)
2K5830 Plug - Oil - drain - (1 in gearbox)
This ione has larger thread and male hex head, used for MGA 1600 and all MGB.
The smaller 6K499 plugs are not 1/2-14 NPT tapered pipe thread, but at least that tap works well enough for thread clean-out. Also (originally) tapered female square drive. For this I made my own driver from a 3/8" to 1/2" drive adapter with the output end ground down to the appropriate taper size (grind to fit). Find more alternate tools in article TS-204 in the Tools section.

Female socket plugs should be nearly flush fit (not quite) with perhaps two threads protruding when installed. The square drive plugs (as original) may be difficult to find as some parts suppliers are now providing plugs with female hex drive. Highly recommend using PTFE (Teflon) tape to seal the threads.

Substitute hex socket 2K5830 2K5830