Master cylinder "gasket" to firewall with integrated bellows seal retainer.

Yes, same here. It wasn't stuck to the firewall at all other than 50 years of probable Road film so I don't think it was ever silicone down or anything. I imagine I'm just going to put that on as is. I could probably use the old foam gasket that came off but I did find one available on eBay for $4 so I'll do that too.