Different butt-slide and butt interfaces

While gathering information for my BREN drawings I've come across two different patterns for the connection between the butt-slide and the butt.
Attachment 124390
The notches and recesses as shown in the picture will definitely not fit each other.
(note: in the image I mirrored the butt to make comparison easier)
Both appear to be later MkI (modified) type parts, but I don't know where they were made or when.
I assume the straight recesses in the butt-slide (on the left) are newer than the curved and wider notches on the butt.

Was this change a simplification/cost reduction that sacrificed compatibility with older parts?
Or was it done deliberately to prevent fitting an older butt to a newer slide?
Does anyone know why this was done (and when)?

Changed for simplicity and ease of manufacture. Originally made so that the butt would only fit one way - the correct way! But quite soon it was realised that only a clown would fit the butt upside down.

Given any butt a real Armourer would easily fit it to any butt slide - which we regularly did. There were exceptions. Only a Mk5 butt was to be fitted to a Mk3 gun

I seem to recall that there was not a part number change for the butt or butt slide. The slight changes were noted in the earliest Miscellaneous instruction part of EMER's