This is a rotational reciprocating mechanism, using a geneva wheel and 2 actuator discs with 2 pins each. The larger disc is the input, and drives the second through a pair of gears with a ratio of 2:1. The smaller disc moves the geneva wheel twice, 60 degrees each time. Then the larger disc moves the wheel back to the original position, 120 degrees backwards.
Video at https://youtu.be/56l4rKLsha0.