My 1962 build Atlas has a tach drive boss that looks like that. Same orientation to the bolt holes, but I do not remember how the machined inside surface looked.
Slick
EDIT:
This is a pic of a spare tach gearbox for my Atlas. The drive end appears to be a match for the machining in the timing cover above.
View attachment 78905
and on the bike ....
View attachment 78909