Product review (5 stars): Great fine!
I never knew these existed.  We bought a 20 year old house, and I'd say the disposal was original.  The rubber was nearly gone on it which looked bad, but smelled ever worse.  I was shocked when I found this on Amazon.  I went and looked at mine, and sure enough it had ridges just made for this to seat in correctly.  Mine is an InSinkErator.  All I did was cut off all the old rubber and snap this in to place.  Worked great.