Do any of your combinations need masking tape to work?Just to back up a bit, the magnet/motor assemblies including the top plate and pole geometry are all the same on all 13 of mine. I bought two of them new when these first came out in the mid / late 80's if I remember correctly, and the last one out the door of JBL not to long ago on the etent sale sight.
I bought seven of them new in all and got three variations. There are two different size phase plugs, one is 1.600" at the base, the other is 1.715". Then of course there is the D8R075 and the D16R2405 diaphragms for a possibility of four combinations, one of which I have never seen from the factory in my new stuff, which is the 1.600" plug withe the D16R2405 diaphragm.