They probably weren't manufacturing new receivers, barrels or other parts, so didn't need the full production machinery. At that time there must have been millions of existing spares available from MoD stores or from the trade, plus BSAs own stocks, plus access to the dozens of sub-contractors in the gun trade. BSA had something like a dozen plants just in the Birmingham area, plus a very large WW2 workforce to hand. I imagine the FTRs were a welcome use of spare capacity in the depressed 1950s....