SM-SH51 are 4degree float
SM-SH52 are 6 degree float
SM-SH55 are zero degree float but multirelease... best for casual users/commuter
They are all compatible with M646/536/424/323/545/515/505/324/a515 ...
the SH52 though often cited as being M858's, That is only because that's the cleat you have to use with
them, but the cleat works with the other pedals.
for your pedals
SM-SH50 (zero float) is the recommended on for the M737/525/323, though SH55/51 will work with it since I doubt you'll find any SM50's
. SH55 giving the same zero float, but the multi-release and SH51 giving 6 degree float in this case apparently
not for M535/747 pedals though the float is 2degree and not zero and 12 degree not 6 apparently
Anyway all info interpreted from Sutherland 6th (737/747 info) and 7th (top part)