Santa was nice enough to bring an 8x10 Eastman 2D by Folmer. From the back, on the lower right is a knob underneath that must have a gear on the end of it. It fits through a keyhole shape in the brass. It appears to control the rear swing. This part is missing. Does anyone know where to get one or know if my guesses are right and know what this thing looks like so I can make another?

Thanks