One major change with the -2 that's rarely mentioned: The recoil spring is retained by a shoulder at the front of the slide, rather than by the barrel bushing.
The 39 could - and sometimes did - launch a broken bushing and spring down range. I recall a trooper saying it did not inspire confidence when he saw that happen to a legendary ISP captain.
The removal of the hump came a bit later. Mine from 1971 still has the hump.
Still later, shorter magazine bottom with a cut-out in the flare at the bottom of the grip.
Then, a one piece barrel bushing.
