Retinas aren't M39 mount. Dunno about framelines in their viewfinders. I don't think any of the near-Leica-copies from Nicca, Leotax, Yashica have switchable framelines (although otherwise they are nice).
If you want switchable frames or framelines in the viewfinder, then the camera is generally going to be bigger than a Leica III or copy aka "Barnack" style camera, because the nice viewfinder makes it bigger. But only a little bigger. A Canon 7 is maybe 6 ounces heavier than a Barnack/Leica copy style camera.
For the OP's precise use case, there is some generation of Canon - probably the V or the L - that has switchable 35 and 50 mm views, and is a little smaller/lighter than the later ones like the 7.
Or, compromise on a fixed 40 or 45mm lens and get a Canonet or a Konica auto S2 - the humble Konica has the nicest view and framelines of any of these, IMO.