compur
Been spending a lot of time on here!
My enlarger as a little red colored plastic "flap" that can be flipped under the lens. Is this a filter? It's for changing the contrast, right?
The purpose is to allow you to focus on a sheet of B&W paper without
exposing the paper (theoretically, anyway) because B&W paper isn't sensitive
to red light (theoretically).
I advise not using it. Instead have a sheet of paper that you only use
for focusing and then replace it with a fresh sheet for the actual exposure.