Have fun with the permanent line it will etch on your stem. ?Yes a propriety rubber. I’ve only installed one thus far, on my savinelli author which has a very wide mouth piece on the stem and it initially the softy did not want to get on there. Being a mechanic by trade, I put a little lube (saliva) on the mouth piece and it slid on easily and fits like a glove. I will try one on a different pipe tonight perhaps, but so far so good.