The "new" filters that i have purchased, and they have been several as I have an R, a ZTU, a ZA, and a ZB, have all been as you describe. However I think you will find if you apply some force to the 'screw-on nut' that it is not attached to the "old" filter. It is meerly seated in the small flange you described. Remove it from the old and tighten it onto the new seating it in the same manner. You should be all set then.
: I recently picked up a "replacement" oil filter at the local NAPA store that their parts book said fit most of the older engine models (R,Z,ZA, etc.) I'm replacing the old one the actually has a screw-on nut on one end and 1/2" flange on the other. The replacement filt has no screw-on nut, but two short flanges on either end. Can anyone tell me if this will work as well as the old one? There were no numbers on the old so I couldn't cross reference it at NAPA.
: The engine I'm working on is from a late 40's or early 50's ZA.