Correct, it the reverse lockout. It has a dogleg shifting pattern, so reverse is where you expect 1st gear to be. There are spring in the shifter to prevent going into reverse, but it is also preventing you to go into 1st. I figured it would then still be very possible to put it inadvertently into reverse at the traffic light, hence I 3D printed the green lever.
i do want to change the shifter, as the springs are making it difficult to put in first gear from my seating position. Takes quite some force.