Correct frog for a No9 Mk1 bayonet?

[browningautorifle]

I think that would be a matter of opinion and what frogs the battalions happened to have. As long as they weren't for the #4 spike they would fit the scabbard and they COULD be fitted, you could find them issued. I would think the ones used last that carried over to the #5 carbine and the FN bayonet/SMG bayonet would be most appropriate.