I wouldn't call it evidence, it's more the information one can find here or there (books, internet). Here is a quote from
Joe Baugher's site e.g. :
The so-called "Cheyenne" tail gun mounting modifications were incorporated in the B-17G-80-BO, -45-DL, -55-VE and subsequent batches. These tail gun mountings also had a reflector gunsight instead of the previous ring and bead. With this installation, these B-17Gs were five inches shorter than the earlier versions.
And the same from the Crowood book with the serials:
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