This is a GM manual transmission flywheel. part# 14018712.
I've had it for years. I was going to use it as the base for a mic stand but never got around to it. Seems like it could be used for it's intended purpose as the teeth are all there. All 142 of them(according to the interwebs). Some surface rust. If you're looking for one of these you likely know more about it than me. 12" diameter.