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This model number of B.S.A rifle has not been seen, by us, recorded anywhere.
Our best guesses are that (i) some design was held in abeyance - production being cancelled, or (ii) that, possibly as a result of (i), the designation Model 12/15 was used instead post-WWII, this being a model number that the public would recognise as representing a melding of the best attributes of the highly successful Model 12 rifle, so long in production, and the much later Model 15 rifle of 'between the Wars' manufacture ( 1931).