Interesting. Photos would help.
The ' Gold Star Buyers Companion' (by our own esteemed Magnetoman, https://www.amazon.co.uk/s?k=gold+star+buyers+companion&ref=nb_sb_noss
), available via Amazon is probably your best reference work..others are notoriously poorly referenced and copy each other, perpetuating half truths!
Contacting the UK Gold Star Owners Club machine registrar (Ian Jackson, [email protected]
) needs to be your first port of call - he can usually verify the dispatch details for frame and engine number (having/having access to factory dispatch records for most Gold Stars), possibly even having a copy of the original engine test certificate (which will detail the engine's specification and intended use). Ian likes to see pencil rubbings of the engine and frame stamping too, if possible, to assist with authenticating them.
Off the top of my head: (and subject to correction)
1. Mounting lugs should have the hex plate outboard..it's what stops the rearsets rotating on the mount - the stud that goes through the mount has a hex portion in its middle (studs are different left to right too). Having no hex plate would make me very suspicious - I looked at a VERY nice DBD in New Zealand about 12 years ago (oddly in Touring trim too) that had rears mounts like that. Those mounts and the style of frame number stamping caused me to walk away..price was premium and the detail didn't support it for me.
2. Seeing the frame number stamping, both in terms of style, font and location, would be a help. Estimated 'fake potential' assessment (given the rearset mounts): "high".
3. That style of Lucas
headlight, with the underslung pilot, was used, but only up to about 1954, on BB machines, or thereabouts. Personally, I wouldn't expect to see one on a DBD.
4. Not sure about the incidence of QD headlight plugs, though it wouldn't surprise me to see one on a Clubmans bike; less so on a. Touring machine. Could easily be wrong, but I don't think I've ever seen a plug on the headlight shell with an underslung pilot (which will surely prompt someone to show that this is an incorrect assumption!).
5. Akront surely not original. Borrani was a period thing; being an Italian-made Rudge item (they bought the rights, or some deal like that in the 1920's or 30's), (will probably have the old 1920's Rudge logo on it, or would have).
On balance, even if the frame's authenticity is not established and the other concerns you mention might be valid, it's likely this is still a great machine to own and ride and looks the part. Price would/should reflect a lack of a frame/engine match or the frames B31 origins (if that's the case). To my mind, a genuine frame/engine, even not matched is worth (there's a statement) more than a converted B31-framed bike, though there's no difference to the way they go/ride. Typical UK values would see a genuine frame with a genuine engine, though not a 'factory pair' sell for 2-3000 pounds less than a bike with a factory provenance and it's original registration plate.
Keep us posted!