Quiz for today--Model H, Off what part did this piece come from??

[AG-6Hguy]

Pulled my differential/trans drain plug and this little guy along with another piece of metal are imbeded inside in hardened oil. Reminds me of last time I was at the LaBrea Tar Pits....
No oil came out so I assumed it was dry as it had not been operated in years. I reinstalled the drain plug.
I had a gallon of gas/oil mix for my chainsaw and poured it in followed by another gallon of lacquer thinner to flush the system. I got about halfway through the 2nd gallon when I heard fluid going into the drain pan. It was going past the seal around the "transmission /engine coupling end"--what I'd call the pinion. So I immediately pulled the drain plug but again nothing came out. Several quick probes with a screwdriver finally broke through the clay like sediment and the goo began to slowly pour out.
Looks like I'll be removing the top differential soon to replace the seals.......

[Bob]

My guess is a brake band rivet, but its steel, so I'm questioning that...