This is the original version of Ro-7.
The manual Žen-29-2 states that this construction was not suitable for wooden AT mines. The wood is flexible and the pin is pushed too slowly into the detonator. Result was as much as 70% of duds.
The Ro-7 II was developed to cure this problem. The spring accelerates the pin.
The original Ro-7 could be used in the bakelite mines.
Both fuzes look the same on the outside.