A simple question (with, I suspect, a not so simple answer). I have a lens with no markings (or suspect markings). How do I figure out its focal length and aperature?

In this case, it's in a brass barrell which someone seems to have modified to add an aperature in the center. The lens elements front and back do thread out if that helps in the exercise. The aperature is somewhere in the middle of its 3" or so length so it'll be difficult bu not impossible to get in there and measure the diameter of the opening.

Right now I have the lens wedged into an extra lensboard of my 5x7 B&J - I need to check again but it doesn't seem to cover 5x7 but does cover 4x5.

Thanks,
Dan