@benjiboy & philosomatographer,
SLR wide angles are mediocre compared to RF ones.
The SLR ones use more complex optical scheme, more surfaces for the light to pass through, while the RF ones are simpler, easier to correct, perform better, have less distortion and so on.
RF can have 100% or more viewfinder coverage, while with SLR that is a challenge if You want a smaller body.
There is no SLR smaller than Leica I or II or even III*.. even the M bodies and lenses are tractors compared to the original Leica I system.
There is no 50mm SLR lens that is more compact than Leitz Elmar 1:3,5/50 and perform so good.
There is no wide angle SLR lens that is more compact than KMZ Russar MR-2 1:5,6/20, let alone being even close to its optical performance.