The Zeiss serial numbering system is confusing at best. And things became even worse after production shifted to Japan. There's a book called Fabrikationsbuch Photooptik III, and it is the best compendium of Zeiss serial numbers I know of, but it is spotty and doesn't show dates for either of the serial numbers you mentioned above. About the only information it does provide is that the 6591927 lens was made toward the end of a batch of 4,000 copies while the 6966837 comes from a batch of 693. Counting all of the batches that are listed in the book between the two serial numbers, there are at least 2,000 25/2.8 lenses between the two serial numbers. However, there are massive gaps of serial numbers in the chart and it is surely incomplete.
I believe, and someone correct me if I'm wrong, that Zeiss assigned serial numbers based on whatever lens was in production at the time, and not based on what the actual lens was. Indeed the section of the aforementioned book that has the chart organized by serial number has far fewer gaps and shows production shifting from one given lens to the next.
I've seen a lot of chrome mount Zeiss lenses, and I always associate them with older models, AEG, though I'm not sure I should do that because I've seen chrome mount MMJ lenses before too, such as the 100/2. Also, the anodized (black) mount lens on the B&S is an AEG. In short, I wouldn't worry about the chrome mount at all. I'd be more concerned about checking the screws for tampering.
Thanks for coming back. Seems a bit confusing but i suppose why should we expect companies to organise there serial numbers so that twenty or so years down the line the equipment we buy has a bit of built in history
Because of our holiday weekend in the UK and the fact that I`ve just had a mail to say the lens is on it`s way from Germany I won`t see it for a few days but i will report back once it`s here.
Wouldn`t it be great if equipment had a registration document like cars where all previous owners were registered in it and maybe a chip to tell us where in the world it`s been and what camera the lenses were attached to
