Since most of the SLRs have meters and Leicas before the M5 do not (with exception of the M4-2 and M4-P) only a few hobbled SLRs can be considered.
Early Nikon F with cloth shutter curtains.
Pentax Spotmatic without batteries or with a broken light meter.
Pentax H1a.
Anything with a vertical travelling shutter is out. A working light meter is out unless we're talking Leica M5, M6, M6TTL or M7. In those cases, a metered Nikon F with cloth shutter or similarly built working Spotmatic would do. Shutters that go higher than 1/1000 disqualify the SLR analogue to the mechanical Leica Ms so no F2, no Pentax LX but the MX would be a suitable analogue to the M6.
Just my opinion.
As for the original question, I'm not sure what it is. Size? Ergonomics? Features? Weaknesses? Lens mount legacy? ... Snip!