120gr and the 115gr Silver Bear that replaced it were what I kept my Russian IJ-70 loaded with. Both worked fine in my commercial Mak (that I no longer have) but I understand many had issues with the 120gr and to a lesser degree, the 115gr as there were inconsistencies in OAL and the ogive was quite flat due to the wide holllow cavity.
I liked them both and still a few of each, but you'd be out of luck if you were wanting some... neither have been manufactured for quite some time. The only Silverbear JHP currently manufactured is 94gr, which is a far cry from the 120gr and 115gr in terms of expansion (the 94gr variety doesn't want to expand).
This doesn't mean the 94gr JHP isn't worth considering. It has a fairly wide meplate, so that if it doesn't expand (and in my experience it won't), you will still have the benefit of a truncated FMJ. If your P64 feeds them, I'd suggest you use them.
There are other options out there that not only are designed to expand, but actually do, such as Hornady's 95gr XTP and Corbon's 70gr Powerball. However, as I feel the only strength my P64 has is penetration, I'm concerned that if it
were to expand, penetration might begin to boarder marginally acceptable. It's already a mild round to begin with, landing in the realm of the .380acp in terms of energy. Personally, I'm content with a truncated profile FMJ for this mild round, especially from a short barrel.
You had mentioned a Taurus M85 for fall/winter carry. I'd say the P64 compares well to the M85. It might chamber a slightly less-powerful cartridge, but you get two more of them which essentially makes up the difference. I'd honestly say the P64 and M85 are near equals in terms of effectiveness.
As you're likely already aware, the quality of the P64 is surprisingly nice for the price. The single action trigger also makes it an accurate little pistol too.
I carry mine in ThunderWear (funny name but interesting/effective concept) and carried it during a two week vacation to Myrtle Beach last year without issue. Heck, I even went on a "slingshot" ride wearing the darn thing, that's how confident I am with that pistol/rig combination.
My 1976 P64 with Silver Bear 94gr JHPs (pullet bullet to the right of target to show profile).
Same P64 with Silver Bear 94gr FMJ (pulled bullet also shown for profile).