As the information seems to be totally absent on the internet, here are the dimensions I measured of the brass drain plug, on a Mk2.
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I believe this is for the overdrive fitted to the Moss gearbox. Keep in mind the full syncho gearbox has a different overdrive which has more conventional hex plugs. Three pointing down, one for the pump, one for the accumulator spring, one for the operating valve and a side mounted one for the cylindrical filter gauze which I would assume would be the one used to drain the overdrive.
Hello,
If I am not mistaken, the type A overdrive was fitted only to the Moss gearbox. The Jaguar all synchro gearbox was fitted with the compact type A.
My drawing is effectively only about the type A and not the compact type A.
Edit: As I was not sure if it has to be written “A type” or “Type A”, I changed the tittle according to the way it is written in the Haynes manual
Hi J-Christophe
I can’t get your picture to load. Could you please try to upload it again? really interested in the measurements to be able to re-produce one for my MK2 3,4 1962 which needs a gearbox oil change.
All the best
Anders
Here is the picture deleted from the first post (disregard the mention at the bottom as it was a sketch sent to manufacture a tool). The lug width is a bit too large according to the feedback I got from the manufacturer of the tool I ordered. He told me that he had taken quite a large tolerance on them and the tool was still a snug fit.
Hello Anders,
The picture is back on line.
Have a nice day.
Jean-Christophe
Perfect!
Saved my day! 
All the best
Anders