The shotgun shell (I have a 20ga. one) indeed DOES sit perfectly flat. The beam certainly appears to be centered and there is an adjustment screw that I haven't fiddled with..
This works very well for me and it was like $12 shipped from California. Now to figure out how to align this Beseler 23c (shoot me). I need to bore out these two screw holes that hold this bracket in the back of the enlarger, you can see that one screw is higher than the other.. thus the head is just tilting to the left. Sloppy.