Assuming bikes were built with sequential serial numbers but not knowing if the first number was R1, R100, or whatever, there were over 5,000 early ones built. The double seat tube ones. I have two - the one below folded is SN 5XXX. Incorrect seat and rear wheel assembly. War Grade front tire, The one to the left of the sign is SN 12XX. All original and weathered including seat. War Grade tires. The silver one is a "late model" with the single seat tube. SN is 77XXX which might be the highest number known. Needs crank assembly, seat leather, and War Grade tires. I won't comment here on what I would sell these for but my guess is the one the OP posted ought to sell for $3,000 or more in the U.S. and maybe more in UK/Europe but I don't know the market there. I am in Northern California. This pic was from a motorcycle and bicycle display at our big military vehicle event and swap meet.
This is a good time to post the differences I know about between a very early bike and the more common early model (although any double tube bike is probably 10 times rarer than a single tuber.
The hinges on 12XX are made from pieces of angle steel, not castings:
This was the standard production set up - castings. This on the very late bike but are the same on SN 5XXX:
It would be nice to know the earliest SN that got the hinges made from castings.
T-bolt with straight rod welded across the top of the bolt head is a very early feature:
Standard method was rod went through a hole in the head and each end was bent up:
An early feature was the pedal arms were not reinforced early. A thick section was added later:
Here is my 12XX:
The 5XXX is the same:
This is how most look, this from a 37XXX SN bike. Pedal driven inboard of course. I don't have a correct crank/pedal assembly on my late bike but I'd probably buy that from someone for the right price.
There may be other differences, I am not a big student of these.
I hope the OP doesn't mind this being on his thread.
John