You'll find some will suffice with a No4 scabbard. Others will accept a No5 scabbard rather snuggly. I don't know if they were stretched for that purpose when newer, or if they were sewn to accept the No5?
The No5 scabbard accepted quite a few later bayonets, so its a tough call to make.
I never tried to force anything into a looser fitting frog. Fear of tearing old leather or popping old stitching etc.
Having said that, IMA used to sell some they called South African. They fit the No4 scabbard OK, but they also fit the "Belgian" style scabbard for their version of the No9 bayonet. The scabbard being slightly slimmer than the British style No5.
Those were relieved on one side of the hoop to allow the socket to drop lower.
The one shown here displays a crunched down distortion in that area as if it had been used with a socket type mount.
Hence my initial hesitation in my first post on the matter.