I purchased a lens recently which has led me in direction I didn't expect. It's an Ilex. In barrel, later vintage (I think--it "feels" '60s or '70s vintage, side and black). No diaphragm. It is marked simply: "6⅜” f:4.5 ILEX PARAGON MADE IN U.S.A." No other markings of any kind, not even a serial number. The rear lens cell has the numbers 164.4 scratched into the outside perimeter {presumably the back focus). A cursory test on the ground glass shows this thing to be *really* sharp even at its fixed f:4.5. Anyway, I have a copy of their Series S Caltar in Acme #3, and took a small gamble on this Paragon, thinking the cells were almost sure to go right into the Acme. Boy was I wrong. Turns out that the Acme seems to have a different thread pitch on the front, than the rear. The Paragon cells are completely interchangeable on the barrel, and the rear cell screws right into the shutter (and so would the front, if the rear of the shutter had enough depth). But neither Paragon cell, nor the rear cell of the Caltar, will screw into the front of the shutter. I don't recall ever running into a situation like this before (though, by the same token, I don't know that I've ever tried swapping cells around, either), or even seeing a reference to it. Certainly, I have observed that some shutters have a different diameter at the front than at the rear. But different thread pitch? Is this. . . normal?