Uhm... Jan... your first statement I think is not
perfectly correct... the code for the folding hood for Summitar was SOOPD... the SOOFM (almost identical) was marketed as the hood for the 1st Summicron 50, which could also accept the late-version SOOPD... (indeed, my SOOPD is engraved both "Summitar" and "Summicron").
It's a tricky question... the original SOOPD had a clamping screw, so it could be fitted to the first Summitars with the "plain" (not grooved) front rim, then, when the grooved Summitars entered, the SOOPD lost the clamping screw and was made with a spring to fit the groove... so becoming de facto identical to the SOOFM (FWIK, the only difference is the engraving... "Summicron" only for the SOOFM)
Anyway, these kind of hoods are a little annoying... fold-unfold... cannot be kept on with camera in std. case... better to use, as you say, IROOA/12585 or the item quoted by Theo : it must be the clip-on ITOOY/12580, made for the last-version Elmars 50 (3,5 & 2,8, not for the old Elmars with 36 diam., who required the clamp-screw FISON).
Sorry for the pedantry but... like a lot these tiny (and mostly cheap
) collectibles...
