Having served as the standard Soviet military sidearm for several decades, the PM Makarov was getting a bit obsolescent by the late 1980s. More of the world was using locked breech, 9x19mm service pistols but the Soviet Union still had essentially a domestic version of the Walther PP. To extend its capabilities, the PMM (modernized Makarov pistol) was developed, and adopted in 1990.
The new design used a double-stack magazines for a capacity of 12 round instead of the original 8. The slide remained identical, and the new magazine used a double stack body that narrowed to a single stack tower for the top three rounds. This allowed the slide and feeding geometry to remain identical, significantly simplifying the design process for the pistol. A new plastic grip was designed to fit the larger frame, and while wider and bulkier, it is arguably a better fit in the hand than the original Makarov.
Secondly, a new cartridge was developed to bring the Makarov up to 9x19mm ballistics. The case remained unchanged, but a larger powder charge gave it aa roughly 30% increase in velocity (from about 1060 fps to 1360fps). This was made possible mechanically by adding three spiral grooves to the chamber to increase friction on the cartridge case, and slow down the opening of the slide. However, this round was never fully adopted because there was nothing to prevent it form being fired in regular Makarovs, with unpleasant results. Thus the grooved chamber design as also dropped from PMM production.
The example I have on camera today is a Baikal IJ-70-17AH; this is the commercial export version of the PMM, chambered for .380 ACP (commercial exports in 9×18 were also made). In addition to the different caliber it uses an adjustable rear sight, which was added to make the gun compliant with import regulations and is not present on the military PMM.
“(…)9x19mm service pistol(…)”
PMM does repeat Czechoslovakian approach of Vz.82 pistol
http://modernfirearms.net/en/handguns/handguns-en/czech-republic-semi-automatic-pistols/cz-8283-eng/
and pistolovy naboj Vz.82
“(…)larger powder charge(…)”
Not only was powder charge altered, bullet was different that, according to https://www.armoury-online.ru/articles/pistols/russia/pmm/ it has steel-core, conical shape of nose (as opposed to PM round) and give muzzle velocity of 420 m/s and energy of 494 J. It was intended to give increase penetration to cope with wider usage of body armour seen in 1980s. There was also developed expanding bullet.
Beyond already mentioned problems with user attempting to use said cartridge in older 9×18 fire-arms, it caused increased muzzle climb, which make rapid fire more challenging.
“(…)slide remained identical(…)”
armoury-online claims otherwise Рама и затвор-кожух усилены и имеют больший, чем у ПМ запас прочности
“(…)extend its capabilities, the PMM (modernized Makarov pistol) was developed(…)”
There was another one, namely OTs-35 https://gunrf.ru/rg_pistol_ots35_eng.html
it also used improved ammunition, but kept original capacity.
“(…)1990(…)”
Bit later, after collapse of Soviet Union, there was another “Makarov but more modern” spawned by PM, it was intended for export and thus named МР-448 Скиф
https://www.armoury-online.ru/articles/pistols/russia/skif/
and also boasts capacity of 12, though it is bit lighter due to using polymer frame. This weapon itself spawned МР-448С Скиф-мини which is similar but smaller and thus has capacity of 8.
Regarding PMM cartridge, there also existed sub-machine gun made specifically for it, named KLIN, see 3rd image from top http://modernfirearms.net/en/submachine-guns/russia-submachine-guns/pp-91-kedr-klin-eng/ which, due to worries about stuffing said ammunition into wrong fire-arm was put out of use.
Must see a comparison shoot between this and the CZ82-83, though I suspect the former would rate as more durable and the latter as more pleasant to shoot, therefore more accurate.
Given magazine logistics, I’d really doubt the use of original mags was something they didn’t realize. Might have been a happy accident they embraced instead of deliberately created, but they absolutely would have realized it and made a few tweaks to ensure it worked.