I am wondering if Rodenstock rear lens elements are usually marked somehow with a serial number or something. I have a 150mm APO-Sironar-N, marked 72 degrees 1990s serial number, that I am 90% sure has the wrong rear element. It's rear element is something like 31mm wide and doesn't "flare" out at all- it's just a straight barrel. It also has no markings whatsoever that I can find. Is there any definitive way of telling what the rear element actually goes to?