I have just acquired a Copal 3, 210mm Docter lens. There is a round spacer, 6mm deep, that fits on the outside of the retainer ring below the lip. When the spacer is in place, the effect is to reduce the amount of the retaining ring's thread that can be screwed into the front element. Without the spacer, the retaining ring will not screw far enough into the front element to get a tight fit with my metal lens board.

What is this spacer for? Is it supposed to make it easier to fit both metal and wood lens boards? Does it act like a washer? Does it matter whether it goes on the front or rear of the lens board?

Thanks.