There was also a Summaron 3.5 /35mm in M mount, 1956-60, code SOONC (same code as the LTM collapsible). I believe this was the first Leitz 35mm lens in M-mount ?
Then the 2.8 35mm Summaron M-mount came-out in 1958 and ran through 1974, code SIMOM.
The LTM 2.8 Summaron also came-out in '58, and ran through 1963. Code SIMOO.
My suspicion is that Leitz thought it would be a good idea to market an LTM version of their "new" collapsible 3.5 / 35mm Summaron in 1956?
Since both the LTM and M versions were dropped after 1960, it would seem that it was not worth the effort ?
Seat of the pants reckoning on my part...
In his Classic Camera Profiles, regarding M Leica lenses, Head Bartender suggests that the 35mm Summicrons were sharper lenses, and describes the M-mount Summaron 35's as "adequate" performers.