I'm also suspicious of the assembly. My Philips PCS130 enlarger uses a reversible lens board. It's mounted so that the lens board extends upward/inward for 50mm lenses and downward/outward for 75mm/80mm lenses. (The amount of "dip" is trivial compared to what's in your photo, though.) If your enlarger is the same way, mounting a 50mm lens would require an even more recessed mount, and then the question becomes: Was the designer a sadist or just plain insane?

My advice: Mount the board in the enlarger as it is and see if you can focus. If you can't, reverse your assembly. Don't worry about the aperture ring until you figure out how it's supposed to be mounted.