And the best explanation I have found so far (a Czech website):
Translated from that website:
In April 1940, during a military tour, was first publicly presented three prototypes of the KIM 10. This show was also attended by representatives of the Soviet government, who watched the incident from the balcony of the government palace. Among them was Stalin and it was he who allegedly sent engineers the message that he did not like the KIM 10 because it had an outdated look. Design changes were made, and after this adjustment, the car got a new type designation KIM 10-50.
Adjustments related mainly to the front of the car. It got a completely new front opening bonnet (the original prototype KIM 10 version was opening from both sides up). The headlights are now part of the front face, in which they are embedded (the original version had the headlights on the upper part of the front fenders).
A total of 517 cars of brand KIM were manufactured from 1940-1945. Just for clarification:
1940: Three KIM 10 prototypes
1940-1941: 500 pieces of cars KIM 10-50 (two-door saloon) and 10-51 (convertible). How much is produced for each version separately is not specified.
1941: Two KIM 10-52 prototypes (four-door saloon). War stopped production.
After the war, in 1945: 10 pieces of the car KIM 10-50 and 2 pieces of KIM 10-52