What is my seal called?

I can't find any posts that talk about a crush sleeve on any 7 1/4 on this site.

Do you have a 1971 factory Plymouth service manual for your car? That will be the best money you will ever spend on your car and the CDs are on eBay pretty reasonable.

No I honestly don't. I do need to get one. Everything has been so simple so far. I never bought one.