I doubt this sheds much light, but in the "show us your drain holes" thread there's this shot of this BB shell on a Ron Cooper frame.
I don't think the OP's is a Cooper but I think somebody also showed a raw BB shell with similar slotting in some other thread more recently (but could have been 4 slots instead of 3) that was made by a Brit supplier and my unreliable recall is it was from Haden...but I cannot find a visual ref to confirm that.
Anyhow I think other Brit frames used such shells, maybe even an occasional rare Raleigh (?)


https://cimg8.ibsrv.net/gimg/bikefor...f6aae6293d.jpg