Stamped Bolt Markings

Got an interesting email from a guy. He's got a WWII prop hub he's trying to identify. The only markings he's found so far are on the bolt heads, the triangle and circle indicating the bolts are close tolerance/high strength bolts.

Here's the interesting bit: The markings are STAMPED into the head, rather than raised like most bolts (forged or molded, I guess).

The only thing that occurred to me was some sort of limited-production bolt that was custom-machined rather than mass-produced.

Anybody have any ideas?

Ron Wanttaja

Got a follow-up email from the guy. He dug further into the hub and found an inspection stamp. From Fiat! So, obviously, non-US practices.

Ron Wanttaja

Many of the fastener specifications allow either raised or recessed marking. Dependant upon vendors practices. Easier to fabricate a "raised" head setting die than vice versa. Again, depends on manufacturer's practices.