Here is a picture of the Ford T9 gearbox seen from the bottom as installed in an MGA. It shows the side fill and oil level check plug a few inches up the side of the gearbox, and a flat spot on the bottom with no drain plug.

This is a very typical oil fill plug. Just fill until the oil starts to run out, then install the plug. In production they know exactly how much oil it needs, so they just install enough oil and stop before it spills. That is also done before the unit is installed in the car. As such, the side plug is primarily for field service to check oil level and top up if it happens to leak a little.

Apparently the intention is that the gearbox will last for the original intended life of the car without changing the oil, so no drain plug is required. Perhaps it is a way to guarantee that no one will change the oil and install the wrong kind of oil during the warranty period. By modern practice, never do any maintenance until something breaks or quits working, then pull it out for repairs or replacement.

It is open admittance that the manufacturer had no intention that the car would last forever. It may have been expected that the gearbox might last longer than the engine or body. When anything substantial should fail, like engine or body (or even the gearbox), the car might be depreciated enough to junk it rather than repair it. Our odd habits of dumping lots of money into a junk car to bring it back to life, or trying to make a car last forever, would be the rare exception to the rule, so there is no justification for the cost of a drain plug in the gearbox at original manufacture.

Once you understand this, you get to make your own decision if it is cost justified to drill and tap and add the drain plug when you put the unit back in service. Incidentally, inside the bottom of the box where you see the round flat area there is a donut shape magnet. If you are going to drill and tap for a drain plug you might consider some effort to locate the new port in the center of the magnet.