Black plastic piece in a slot on bell housing starter rim?
 

Or maybe it's rubber. It's so perished, I can't even tell anymore. It just fell apart as I slid it off. It slides into a square cut-out on the rim of the transmission bell housing, right where the starter is. What's it for and what is it called, so I can get a replacement one.

no idea what its for, it is rubber though

mazda calls it a plug blind part number is 8540-16-241A

Cool, thanks.

It's an inspection plug.

I threw it away. Mine was broken.

for what?

Yeah, inspection of what?

Also I checked with Mazda, and they say it's discontinued. So what options do I have now? I don't want to just have a hole there, dirt and moisture would get into the bell housing. Should I just make my own plug and close it off?

yeah just make your own plug