I have been sampling classic cameras for a little while, and I am curious about variations of favorite cameras. I am not referring to different configurations, like alternative viewfinders, but simply updates or changes made to the the same model. I would imagine this applies more to a long running model that was revolutionary to the market at the time. My favorite example is the original Nikon F. Uli Koch's book and the online Nikon F Collection and Typology reveal several variations of this camera as it evolved towards the F2. For the Nikon F, I would imagine it would would drive a collector crazy to find every example of this camera's external or functional changes, all the way down to the flash contact post insulation (white, black, square, semi-circular). I like to recognize variations of the late model Nikon F, but of course, possible user manipulation has to be considered as well. Learning about serial number milestones is very interesting as well. Certain cameras have a flaw up to a certain serial number before it was improved, and its good to know that. If I do not find a table anywhere, I will tally serial numbers I find on ebay or online for a specific camera of interest, and note the configuration of that serial number. I would like to ask the CMC community if specific camera models of any brand are well known for its variants, or if one captured a reader's attention for a while. Of course, we all have different interests, so one person may be fascinated with a wide range of variations, while another reader may only be interested in the significant change of a one feature, like the M3 single stroke vs double stroke film advance lever. Do you have favorite references for a specific camera, like Koch's trilogy? I would imagine that the evolution of a brand's prime lens may have an interesting evolution as well.