Yes, the two size rings are for under the stainless tub, actually one was for using metal #10 cans, the other is to fit the stainless tub that comes with the warmer.
SOME of the GM warmers were shipped with a metal SPRING, Kinda like a bedspring. if you get one of those DO NOT USE IT.
I know of a case where they had the 'Spring' and while using it, the hold down collar on top moved and let the pump POP up spraying everyone with hot water and butter!
fortunatly the temp was not set TOO hot, no burns but scared the heck outta everyone and made a real mess.