It is the back of the crank shaft. There is nothing but solid metal in that location. No oil passages in a Stovebolt.
Could it be that it is not actually a plug and just a step machined in it.
I don't know about H-Matics, (so@@y about the ab@eviations, but my key between "E" and "T" quit wo@king), but P-Glides have a diffe@ent sized hole in the back of the c@ank, needing a special pilot bea@ing.
Ca@l
(New keyboa@d will be in on the 26th.)
