Hello JP,
Welcome to the world of the TR3B, better known as "DOCUMENTATION? Fuggedabouddit!"
The guys on here can really give you some good advice. I particularly appreciate the responses to your post, as I did not know that the TRA's build records do not include the Bs, as they were assembled elsewhere. BTW, the "F" in TSF and TCF stands for Forward Radiator Company, as that is where they were assembled, as the factory was going full-bore assembling the TR4.
One of your responses was very good in suggesting "hiding places" for your original color, but not infallible. On my B, those places mentioned were all painted over. The PO gave me photos of the restoration, including the complete disassembly and repainting of the tub and panels. (The car now is red with black interior.) The PO told me that the body guy (a Brit, BTW) who did the repaint took it down to bare metal, and went thru a series of 3 or 4 colors, including yellow! He said that the last color he hit before bare metal was black. So, I was "forewarned" what to expect on my BMIHT Certificate. Sure enough, the car left the factory black with red interior. From the certificate data, I even got the name of the ship that brought it to America, and found the ship on the Internet, and have 3 photos of it in my Triumph folio.
Even my commission plate and body number plate were removed for repainting, so there is not an INCH of black on the car anywhere! (Any black is oil leaks.)
Getting back to your car, the deeper you dig the more you will find. Whether TSF or TCF series, it was built with TR3A parts, TR4 parts, and maybe TR3 parts, if it was necessary to get the car out the door.
Then, of course, the Dealerships enter the picture. Besides the normal dealer-installed options, they were not above switching out interiors, tops, tonneaus, and sidecurtains if it would provide a sale. Which would explain how the last car on the lot was a yellow car with blue interior, black top and white sidecurtains.
Good luck, and enjoy your TR. Mine is my second childhood!
Phil
P.S. Re-reading this, I thought of something else. I read somewhere that ALL TR3Bs came with WHITE tops and side curtains. I believed this, because my black top is a replacement and my black sidecurtains are WHITE underneath and re-colored black. Then, in Bill Piggott's last book, I read that the Bs came with either white or black tops and sidecurtains.