Here's an interesting collection of bits, which I believe would be a great start for a side-car friendly veteran Doug. It depends on dating of course, so comments on whether the machine would get a Pioneer Certificate as pre-1915 would be welcome. Out here in Australia the veteran date is the end of 1918, about which I'd say there is little doubt.
The frame uses "D" section front down tubes (were these used in 1914 only?). The only number I can find on the frame is the numeral 4 stamped on one of the machined faces of the engine mount. (I'm not having much luck with ID numbers on bike parts lately!) It is in generally pretty good order, other than a blacksmith's repair to the bottom of one of the downtubes (the flat steel lug inside the D tube has broken clean through) and a lump of bronze where one of the engine mounts attaches to the bottom frame rail. Both look to be straight-forward repairs. There seems to be no problem with rust.
The motor is stamped 1753 on the flat surface where the magneto mounts. The dating page
https://www.douglasmotorcycles.net/aa-files/html/identify-part1/veteran3.3.htm gives 1888 for the last number for 1914. Not sure what the Sunbeam Club thinks, but check if a 1914 date is important to you. The motor has vertical fins only on the top (i.e. "up" side) of the cylinder head, which I believe from other posts means the motor is a 3 1/2? If there is any doubt, I guess I'll have to lift a cylinder and measure. The motor seems to be in generally good order, but for a number of broken fins (as usual).
The gearbox is, I believe, a 3-speeder from a later 4 h.p. In keeping with my numbers problem, I can find nothing stamped on it other than the matching fitters numbers of 76. The correct box is, I believe, two speed.
So that's it - good bones but plenty of parts to find and work to do. The tank is not as tricky as some, since the oil is carried in the sump and not the tank, and I assume wheels and other parts are not too different to 2 3/4 parts. Overall, however, this machine is quite a lump compared with the 2 3/4 - just the thing for a smooth purr down to Brighton. Subject to dating, of course - I'd be interested to hear what people think on that subject.
Comments and questions here; send me a p.m. if you want more photos or wish to make a serious offer. The bike is in Australia, but I would consider shipping elsewhere if you're prepared to cover the costs of crating and freight. No offer necessarily accepted. No need to rush as I will consider offers before deciding what to do.
(There is also a 1914 2 3/4 project that i will post in the next day or two.)
Cheers
Leon
