Directly interchangeable............ Thinking out aloud, I don't recall so. I think that you have to adjust the kidney shaped protrusions/anti rotation blocks at the front of the butt so that they fit in to the smaller(?) shaped holes in the butt slide. Nothing major of course but........ It might be applicable one way but not the other. But this was mentioned in the old Bren EMER's/Notes for Armourers for the information of those who still have Bren guns in their charges. It suggested that the butt was fitted over the return spring tube and tapped with a rawhide mallet so as to slightly mark out the differences between the kidney shaped slots. Then make off to suit, then fit.

Make sure that when you do fit a new butt that its the front end of the butt INSIDE the housing that takes the weight of recoil and not the edges around the outside of the housing. If not, the edges will split and you'll have to patch it

But once you have overcome the actual fitting to the butt slide, then the butts as an assembly are interchangeable. Not something we'd do in service because after the late 70's the older butts were obsolescent and NLA (no longer available or NP (not provisioned) in the VAOS lists. The only butts available thereafter were the lightweight Mk3 gun type. That's why you will often see old UKicon spec Mk1 and 2 Brens and DP L54's with the Mk4 lightweight butts. Strangely, the curved Mk1 carrying handles were available right up to the end of its life. They were all marked JI, presumably Canadaicon supplied a zillion years worth of spares.