M model (top) pistol has a longer compression tube, and shoots faster; so a shorter "lock time":
From PA article linked above:
"Difference between the IZH 46 and 46M
There is really only one significant difference between the standard IZH 46 and the more powerful 46M and that is the length of the pump mechanism. The 46M pump tube is almost an inch longer than that of the standard 46, resulting in a longer piston stroke that compresses more air. The result is a muzzle velocity of about 500 f.p.s. over 425 f.p.s. for the standard model. Other than that, the two airguns are identical.
Power
The thing that sets the 46M apart from other single strokes is its power. It develops a true 500 f.p.s., with lightweight lead pellets. That’s 75 to 100 f.p.s. faster than other single strokes. The manufacturer claims 410 f.p.s. for the 46 and 460 f.p.s. for the 46M, but I have done extensive testing over 20 years and have found my numbers to be more representative.
Accuracy
You have already seen the stunning accuracy of the MP532 air rifle. The IZH 46 pistol also has a hammer-forged barrel that gives similar accuracy. It rivals the accuracy of vintage target airguns like the FWB 65."