The comment about "the great bulk have been sold into the sporting rifle market" suggests that this included those with the better barrels.

Would be an open question as two whether new-made barrels were fitted, but I'd suspect it was a case of selecting(?) the better barrels from among the many spares that were probably sold off at the same time. "New B.S.A. target-quality barrels", could be read to just mean a barrel that was new and of "target quality" in the opinion of someone at BSA.

Wouldn't even be surprising if rifles were built up with new wood also, as we know from the film "Wood Goes to War" that new P14 stocks were made made during WWII.

I can well imagine the ever-frugal gun trade buying up the cheap left-overs and reworking them into something for which a premium price could be charged, and £ 17/10/0 in the mid-50s was about two weeks wages according to this calculator.