John R,
C15T frames were manufactured in both the early 'swan neck' type and the later Victor type. I have an ex-works bike bought many years ago from a friend in the Silsden clan. The engine and frame numbers are correct for year according to the original log book.
However the gearbox has GB40 internals that have been modified to fit by reducing pinion widths because the internal width of the box is narrower in the early crankcases. The gear thicknesses have been modified by grinding the end contact faces. The distributor type points assembly has been replaced with the in-timing case type and an early capacitor discharge type ignition system was fitted. (Now Boyer)
The front fork looks standard but has the late double damped internals. The frame is 'Victor' pattern, not Swan Neck as it should be for the year of manufacture declared. Both wheels have Bantam hubs rather than the larger B and C series fitted as standard.
Both frame and engine numbers are correct in accordance with the registration number in the BSA factory sequence.
I am currently doing a rebuild because my son loves riding pre-65 on his C15s, and would like to ride next years Scottish Pre-65 on a real bike that has already done the Scottish at least four times when a factory mount. The bike was 'on-loan' to a number of the BSA 'Trials' dealers in it's early life and has probably been to Scotland more and was certainly used in many local and national trials. BSA eventually retired the bike in the mid-70's.
All the bits in the bike are genuine BSA, (including the seat pan which is from a C15S), but what I think this shows is that if the original declared owner of a bike was the factory, take it to pieces and check everything in the build very carefully, because what you have is very unlikely to be standard and may have some one-off mods, viz my gearbox.
John R