Hi,

That's the base of the flatbed? They are the markings that represent the draw of the front standard, with the two knobs used to focus the camera.

Turning the two knobs will pull the front bed forward or back depending on the focus point chosen.

Numbers ranging from 130 to 240 represent the most common lenses used in 5x4.

So to use a 150mm lens, draw the front standard out so that the 150 mark on the scale is reached.

Any pictures of the back scale you mentioned?

Mike