De vere 504 lens board 50mm hole.

De vere lens board 50mm hole.

The file "tight fit" is to be used if you haven't got a nut. You can file it carefully and screw the lens tight, maybe use some thread lock.

The other file should give enough space for the lens to move in freely and is to be used with a nut.

Don't forget to cut a small bevel like on the picture.