Yes, from the Hepolite catalogue:

M20 comp.: 1 19/32"
M21 comp.: 1 7/32"

They call it the 'compression height', distance from center of the pin to top of the piston.

Surprisingly the difference is 0.375" / 9.53 mm instead of the 9 mm as you indeed would expect.... So it looks the top of M20 piston gets 0.5 mm higher up in the bore than the M21 piston.

BR Michiel