Used Leica R-Mount Zoom Lenses: Vintage SLR Era

Yes, via the Leica R-Adapter L (or third-party R-to-L adapters). The R-Adapter L is a mechanical-only adapter: R lenses are manual-focus and have a mechanical aperture ring, so no electronic communication is required. All R-zoom focal lengths and apertures work normally on SL, SL2, SL3 bodies, and on Panasonic Lumix S and Sigma fp/Art L-mount bodies. Image quality is preserved at full sensor resolution. This is a common second-life path for R zooms: buy a $300 Vario-Elmar-R, mount on a modern body, get genuine Leica rendering for fractional cost.

Partially. The Leicaflex, Leicaflex SL, and Leicaflex SL2 bodies use the original R-mount but predate the 1-cam / 2-cam / 3-cam / ROM coupling evolution. Pre-1976 Leicaflex bodies are compatible with 1-cam and 2-cam R lenses (which carry both the original Leicaflex coupling and the later R coupling). Most R zooms manufactured after the R3 launch in 1976 are 2-cam or 3-cam, and these mount and shoot on a Leicaflex but the original Leicaflex meter coupling may not engage on later 3-cam-only lenses. Verify cam compatibility before buying if you shoot a Leicaflex.

Three reasons. First, demand: the R system has been discontinued since 2009 and there is no current Leica SLR body, so the buyer pool is mostly adapted-use on mirrorless bodies. The M system, by contrast, has continuous current-production bodies driving demand. Second, supply: Leica made more R lenses than is widely realized: the system was in production for 45 years. Third, the SLR market overall depreciates: 35mm SLR lenses from Canon, Nikon, and Minolta of the same era trade at similar fractional prices. R-mount inherited the SLR depreciation curve while M-mount stayed insulated by its unique rangefinder-form-factor demand.