The Super-Angulon lenses by Schneider Kreuznach are simply superb. I have a 21mm F3.4 M lens for my Leica MD with 21mm finder. In cities, it’s an amazing lens to use. No focus needed, just stop down to f8 and use the Hyperfocal method.
When I started using a pair of Leicaflex SL bodies around five years ago, I bought a 35mm f4 Curtagon P.A. Shift lens. As my meters don’t work, the ‘cams’ issue doesn’t affect me at all. Now I’m thinking about buying a 21mm f4 R lens to use with my pair of reflexes. I’m really glad I changed over to Leica and Leicaflex a few years ago, the gear is amazing. A quality not seen today. I have two M3, the MD and the SL bodies. Leica have started re-using some old model names: my MD dates to 1965, yet there’s a digital M-D. Also there’s a Leica SL and SL2 digital cameras too. I bet the latter are in the landfill before my pair of Leicaflex SL bodies (1970 & 1971) are.