I think that was more to do with conversions. They used to get up to all sorts of tricks to convert section 1 to shotgun cert, i.e. extending and smoothboring barrels.... you used to be able to get a bren gun on section 2 but I think that has gone by the board now.

These days if it started life as section 1 it remains there from here to eternity.

I would assume the many lee enfields that were converted had to have their magazines restricted and welded in place when the 3 shot rule came in.