Hi
Like the images in the link above, I have a reprint of an old (pre WW1) BSA catalogue and your carbine looks to me like their no. 3 'Trade Pattern' with perhaps a later, replacement bolt. All the carbines pictured have a flattened bolt in the catalogue. The original price is marked at 112/6...
However, BSA would fit their 'patent safety locking device' to any service pattern of BSA manufacture at an extra charge of 7/6... so perhaps the bolt set-up and sliding safety are original to this rifle.
Looks great in any case!
Kind regards
Mike