By the way, that plug was used by Nissan from 1966 to 2001
* filler plug 4-speed & 5-speed
* drain plug 4-speed

i believe it is a standard pipe thread part (british thread or metric thread I do not remember)

* drain plug 5-speed uses a different part with metric threaded bolt & washer used by various Nissans including 510 and Patrol

Be careful when tightening. As you discovered removing it, it is all too often over-tightened. On the 5-speed this can crack the adapter plate. The 4-speeds don't have this problem as the drain plug goes into the stronger extension casting