I'm wondering if it's from the bellhousing to engine. I need to do a thorough look see up top when it warms up a bit outside.
For what it's worth, the transmission to bell housing bolt I suspect is M13 x 1.5. I threaded an M12 x 1.5 in there and it was noticeably loose. I found an old parts manual on the Cruiser Cult site. Super helpful. Thanks @red66toy for hosting that resource. Putting the link here for future reference.
I think (and lets be real careful with that understanding) that the markings on the bolt you posted are for an SAE grade 5. That makes me think the bolt was floating around in the cab and migrated in from the top.
Finally feel like my welder is earning its way. Poor two-piece driver's door was splitting in so many places. Ran about 30 inches of practice spot welds before I manned up and tacked the door.
Turned out pretty good. Even impressed the Prospector. He welds better than me, but is out working this summer. And I got tired of watching the cracks grow.