Have an example of both, couldn't fit the rifle one to the carbine. IIRC the carbine bayonet came from Dennis Ottobre.
Agree there is little that can be found about the difference. Anyone else with a carbine (confirming MRD 16.5mm) that can chip in?
A couple of photos of carbine #A130 with mismatched bayonet #A398, showing the comfortable fit at the muzzle. This bayonet has a MRD of 16.5mm.
It is an extremely loose fit both to the M1886 long and short rifles, also the M1885 Guedes, which my other 1886 bayonet ,MRD 16mm, fit a treat.
The inscription on the spine of both bayonets is clear.
I suspect JF's may be for the carbine given the letter prefix and that it appears to have the larger MRD, which IMO would be a plus.
IRRC the 17.4mm MRD examples were altered to fit the Gewr.1888 Commission rifle.