….what’s it for??

 

Fortunately I’ve never had to dive quite that deep into the transmission myself, but iirc there’s a detent spring and ball underneath that plug that retains one of the shafts of the transmission.

 

Might be just an artifact from 4 speed boxes of days past. Spings and ***** are under the fasteners between the two sheet metal plates - where access to the shift fork roll pins is achieved.

Better to return it elsewhere, but we've used it for oil coming back from a cooler, vague recollection it was 3/8" bspt.