If enough time passed already, after completing each car you have small chance of getting "special" car instead, exactly "0.05", which I belive it to be 5%, not 0.05% (So basically every 20th vehicle leaving your factory). Years required are specified below.

Name : Tank
Year that starts appearing : Y25
Chance : Probably 5%

Name : Pikarus 31
Year when starts appearing : Y34
Chance of car appearing : 5%

Name : Tarball P50
Year when starts appearing : Y37
Chance of getting the car : 5%

Note 1:

Game is ending around late Y79 (game still allows you to play, just without setting higher score), consider making manual save around Y70, just in case if achievements are locked after that time too.