This is an older (I think) Beseler 23C2 head that was with some other enlarger gear that I bought. My question is concerning the sliding device on the side of the housing that holds the lens board. It slides left to right, and it has what looks like a piece of plexiglass that opens or closes the light source. I have two other 23C2's that don't have this device. So, the big questions is; what is this mechanism for?