All the different parts, such as butts and bipods etc were made to be fully interchangeable as assemblies. If a production/assembly lkine at, say Enfield, producing Mk1 guns was short of Mk1 butts produced by BSA at Birmingham, then thay'd just get allocated a truckload of Mk2 butts made by 'Tibbies' at Ipswich. It is quite common to see Mk1 guns with Mk2 parts.

And don't forget this. Because a piston or a barrel was designated Mk2 or a butt plate designated a Mk3 it doesn't mean that it was for a Mk2 or a Mk3 gun. It simply means that the part was a Mk2 PART.

You're right about the butts. The Mk1 butt was just an exercise in making space for fresh air! The idea of a gun buffer in the butt AND a recoil buffer in the butt slide was a total frill - if not a complete fraud! Everyone knew it and the Mk2 and 3 gun butts did away with the gun buffering system. But production of the time wasting Mk1 butt continued because the licensing agreement dictated that the gun was to be produced to the drawings.

And like the No4 rtifle, I cannot ever remember an Armourer waiting for an X part for an X gun when there was a tray full of Y parts on the shelf. They might have - but I was asleep that day if it happened