I'm not sure if this has been covered on here before, but it's worth a look. Ebay number 380538102452

1939 350 KB26 engine would i think have KB24 frame.
No BSA i'm aware of had KB21 frame number.
( The 250 KB21 had a frame KB20)
Maybe the lighter 250 frame got used & uniquely stamped ... don't know.
Nicely detailed machine though isn't it, I'm quite a fan of mk1 concentric as well & don't mind seeing them on older bikes.
A post war gearbox.

I don't think its a great investment at that price but looks fun.

It's not clear from looking at the pics. but from the angle of the primary chaincases it looks like a C11G 4 speed box..They are basically plunger A10 boxes in different cases and are far superior to any pre war type. However, that can make it very awkward to get at the points unless the box is dropped in the frame by an inch or two. Also it makes a bit of a nonsense of the 'level plug' in the cover....
One thing's for sure though...it will be a very light bike and if the engine is well tuned it will be a nice ride...Can't help thinking a tuned/enlarged B33 motor would make life even more exciting and is far easier to get parts for...but I guess then you would lose the 'prewar' feel..I agree regarding the Mk1 Concentric...they are cheap, easy to tune and don't look out of place to my eye. Ideal for a 'special' or as a more economical solution for a bike that is being used but isn't 'bog standard'...Ian

email (option): ian@wright52.plus.com

Close up shot of headstock shows a spacer under top yoke suggesting the frame is not correct for the forks. the forks are of the design for a 350 so it further implies a 250 frame being used.
Also i was checking the shot of the lefthand brake plate & noticed the forks have not only been extended at the axle plate but also the wheel has been moved back slightly for trials steering possibly.
The lower wheel position has meant lowering the mudguard by adding spacers to the mudguard mount at the bottom yoke also.