Andrew, according to John Knibbs in his book "The Golden Century" , BSA made the switch from hand cut rifling to mechanically swaged rifling in mid 1959. It reduced the cost of manufacture considerably, and meant that barrels were harder due to the hammers forming the rifled barrel around a central mandrel. This is different from button rifling, where the "button" is pushed or pulled through the drilled bore.

Lakey