Exactly that, BSA designers would make a new change to an existing gun, and that change would be brought in only when stocks of the old components had been used up. The first transitional models did have the MK 1 Pillar rearsight and tap, and once those older components were used up, ( John Knibbs suggests GC4900)they then introduced the newer type fitments.
This gun for sale( now sold) has the early Mk1 cylinder with etching, married onto the new design breech block and parallel barrel, and also has the MK2 flip up rearsight.
As these transitional models are uncommon, and even rarer in good condition, they make a superb addition to any dedicated airsporter/BSA collection
Lakey