Well, after doing a bit of research myself, I ended up buying four "
threaded stud gliders", 1/4"-20, 1.5" long, from Herbiesaudio.net. We'll see when they arrive if they do the trick.
Then, today, I got a phone call back from Pierre at Mapleshade, to whom I'd also sent an email inquiry about isolation feet. He's a serious experienced perfectionist professional, and advised me that their brass "
threaded heavyfeet" footers are what I want. He also said that the "gliders" from Herbie's are very inferior if vibration isolation is the objective, and that point shape is the far superior proper design. He also said his brass feet are much superior to the original steel Billy Bags feet.
I ended up ordering four of the "
threaded thick carpet heavyfoot" brass feet with 1/4"-20 stud (for the pre-threaded Billy Bags rack). These are about 3x the price of the Herbie's gliders, but I suspect they are what I will be keeping and will be returning the Herbie's feet.
In any case, I think I've now solved my problem of the misplaced original Billy Bags feet (although I still am holding out hope that they may actually be in one of the cartons in my big pile where everything was supposed to be placed.
In the same envelope with the Billy Bags feet I also placed the six isolation pins for under my two floor-standing Thiel CS3.6 speakers. So those, too, are currently misplaced and presumed lost. But fortunately Thiel is still in business and I was able yesterday to order a complete replacement set of six Thiel pins, so that crisis is also now resolved.
Both cases closed.