Stamp decoding on engine.

[THE "10" SILVERADO]

Hey guys,
After all summer of waiting to pull my old 350 I finally got around to it. I know the motor was not original to the truck but I'm trying to figure out if there is a way to find out what the motor was originally on. Besides the code on the back by the bellhousing the only other stuff I found was on the passenger side of the block (not by the alternator like I've read) and it looks kinda like a clock, I know that sounds weird but it has an arrow in the middle and little lines that look like a clock. I have no idea what this is so I was wondering if anyone could help me out and figure out what it means.

Thanks,
Michael