The 518 are notorious for decay and may just have more demand? BTW I was not trying to thread crap, I am just too cheap to pay $100 for feet. That place makes a nice product (actually a ton of stuff inc non audio) and is a small biz built in USA, well worth the support.
I have tried to find any scientific reason to not use vulcanized or perbunan rubber for a fraction of the cost. I also have a TT shelf bolted to studs and a poured concrete slab floor. Not a ton of vibration. Also my hearing..... maybe aging has benefits?