I think I have solved the "supply problem of perfect 55mm retention ring". I ordered 15+ different UV-filters from eBay and also 5PCS of Amazon Basics filter in 55mm size. Hopefully no-one doesn't anymore need to buy Leica E55 ElPro just for retention ring anymore...
The most reliable source seems to be filters, which have text "55mm UV FILTER" and are like this (this is one of my orders, good deal of 4 filters for 9.95EUR w/ free shipping): link to eBay to MENGS 4Pcs 44mm UV Slim Lens Filter For Canon Nikon Sony Pentax Olympus Etc
I have these same from other vendors as well (plastic box might be different but filters the same). This far all of these have been optimal 49.8-49.9mm inner diameter range and retention ring has nice thickness (many filters have too thin retention ring) and inner edge is beveled to make potential vignetting smaller.
There were many suprices, for example that Amazon Basics I first ordered one and it was perfect and ordered 4 more, and all new 4 are having inner diameter ~50.1mm and are not able to hold OptoSigma 55mm lens in place.
Majority of cheap eBay UV-filters which have only text "UV 55mm" in the ring have "perfect" 49.80-49.90mm internal diameter. However some of these have 50.5mm inner diameter and are useless.
Most filters which are some "brand" e.g. Kenko, JJC, etc. have retention rings, which are too big to hold OptoSigma 50mm in place.
I have never needed 55mm UV filter in my life, now I have >25 of them
