MGA 1500 and Twin Cam
6K499 Plug - Oil - drain and filler - (2 in differential)
6K499 Plug - Oil - drain - (1 in gearbox)

MGA 1600
6K499 Plug - Oil - drain and filler - (2 in differential)
2K5830 Plug - Oil - drain - (1 in gearbox)

These plugs have 1/2-14 NPT tapered pipe thread and (originally) tapered female square drive. Difference in the two part numbers is in size of the square drive. For the part number 2K5830 in the later model gearboxes I think you can use a 3/8-inch square drive socket wrench directly. The earlier part 6K499 has slightly larger tapered 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 modern replacement parts, as most of the parts suppliers are now providing generic hardware store plugs with female hex drive. These can also be found with male hex or male square drive to be used with an open end wrench. Highly recommend using PTFE (Teflon) tape to seal the threads.