During the course of an EBay sale which Ive since had to cancel and refund, Kerry Thalmann determined that my Fujinon 360A lens was fitted with a Fujinon 300A rear element. The lens had been previously sold to me in that configuration without my knowledge back in September 2004. This is not an obvious issue to spot since the 300A and 360A share the same shutter size (Copal 1) and rear barrel diameter (54mm), and the 360A performs passably with the 300A rear element installed.
Kerry has guessed that a previous owner (who owned both my 360A as well as a 300A) accidentally swapped the rear elements, either while mounting both lenses or possibly experimenting with convertibility (checking whether the lens could perform as a 300/360mm convertible). If this was the case, then someone out there owns a 300A with a 360A rear element. I suspect a lens in this configuration will not perform optimally, and we should get in touch to exchange rear elements.
Per information from Kerry there are three tests that can be applied to see whether your 300A has the correct rear element. The first and easiest test is to check the rear element serial number. If the serial number is 780xxx, then it is likely the correct rear element. If the serial number is 521xxx, then you most likely have a 360A rear element and we need to talk! Secondly, if your rear element is 28.1mm long, then it is likely the correct rear element. If it is 34.5mm long, then you likely have a 360A rear element. Lastly, if your rear element weights 106 grams, then it is likely the correct rear element. If it weighs approximately 141 grams, then you likely have a 360A rear element.
Please check your 300A and email me if you have a 360A rear element so that we can arrange a swap. My 300A rear element is in excellent condition (the glass is pristine with no dust, scratches or cleaning marks).