Size plug on Velvet drive transmission with Ford-Lehman

I have a Ford-Lehman 135 with a Velvet Drive transmission
Model 1017010 Serial number 13274 (or A) Ratio 2.10: 1

I installed an oil change system (Groco) with three valves for the engine, generator and transmission.

Where does it connect to drain the transmission?
I suppose on the starboard side there is a plug.
In this case, what is the size (diameter) of this plug for the connection?

Of course I could have my answer by removing the plug, but I should remove the oil before!

I asked Bob Smith, but I have not had a reply.

Thank you for your help!

There should be a plug on the bottom.
That is where you will get all the oil out.

Did the same thing on my CAT 3208 Twin Disc 506 The plug was on the bottom where an oil drain should be.

SD