Different butt-slide and butt interfaces

[erik3D]

While gathering information for my BREN drawings I've come across two different patterns for the connection between the butt-slide and the butt.

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)?

Information

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