The popular belief is that the first polymer pistol was HK VP-70. It is true and not at the same time. If we are talking about a production model, then yes, HK VP70 is the first one (introduced in 1970). However, if you are asking the first ever made working polymer pistol, then the answer is Russian TKB-023 (ТКБ-023), which is essentially a Makarov pistol with a polymer frame (designed in 1963)!

in 1963 Ministry of Defense of the Soviet Union ordered the arms designing bureau TsKIB SOO to modernize the Makarov pistol with the main goal of making it 100 grams (3.53 oz) lighter. The prototype pistol was code named TKB-023.

First attempts were to make the TKB-023 frame out of a lighter steel alloy. However, these proved to be non-reliable. The frames were bending and causing malfunctions and had very low corrosion resistance characteristics. Next material they used was a titanium alloy. The titanium frame seemed to be good, but it was too difficult and expensive to manufacture. So after a number of trials and errors, the designers decided to make the frame out of a moldable polymer called КС-30.

Despite of promising results and achieved weight goal, TKB-023 was never adopted and the project itself was canceled. One of the reasons is that the technology of polymer molding and press forming wasn’t developed enough in the Soviet Union.

https://www.thefirearmblog.com/blog/2017/01/31/worlds-first-polymer-pistol-makarov/