It’s basically just a spacer ring that is used to build up the flange when new flooring has been installed for example and results in the flange being at or below floor level. It is also sometimes used to help rotted out flanges (especially where the bolt carriers are broken/rotted and will not hold a bolt anymore). They’ve been around for quite awhile, but alot of guys don’t like using them and prefer fixing it properly though these rings can many times save your day and can last for years.
One thing I should add though…..I just noticed the original poster specified Marvel rings in 1 1/4 and 1 1/2″. ??? I don’t think a flange ring is what this guy is wanting after all. Unless he is referring somehow to the bolt slot diameters (which still would’nt make much sense).