Quote Originally Posted by sdivot View Post

Question: What's it for? I don't know why you'd use waterhouse stops when you have the aperture ring/blades built in. Forgive my ignorance. Can anyone shed some light?
That lens was for a large 'process' camera. The red 'numbers are actually the diameter of the iris opening (in mm). The slot was/is used for the insertion of filters for graphic arts colour separations.

I just got my 'new to me' Apo-Artar mounted onto a 3/8 Baltic birch plywood lens-board for use on My Burke and James