I have a Sinar Selected Rodenstock 75 f4.5 Grandagon-N, which I bought second hand. It appears first class, with no marks and perfect optics. However, it also appears to be a well loked after older lens (judging by the fairly matt, old look to the barrel). Are there any serial numbered batches of those prone to seperation. Can you age Rodenstocks as you can Schneiders from serial numbers? An guidance would be very handy!