How well do they work? I'd use them in a very full choked Lefever 20 gauge for quail.
Any loads Alex and Morris and crew crank out are tested to the 9th. power before offered to us-- I have been using their 12 and 20 loads since 2009, 100% satisfied. I have only used Poly-Wad spreaders in 12 gauge- in my re-barreled Smith 2E live bird gun- Full and Fuller- 32" ventilated rib- The Poly-Wads in the right barrel do offer a closer to modified pattern, but in my book- OK for SC, but for real pigeons and pass shooting, I like the F&F tight patterns.
Can't speak for quails here where I live in MI-- but I would not hesitate to use RST spreaders in your vintage LeFever 20 gauge. Years ago, when I had a good grouse dog and some productive coverts, I owned and shot a DS LeFever 20 bore- 28" DT- best grouse gun I ever owned, wish I had kept it--