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?
X marking should be portuguese inspection, normally the MRD is about 16,2 but could be little larger after heavy using, there is 2016 discussion by Bo.A. about 17,4mm MRD pieces. Barell ring should be measured from blade side, both orientation horizontally and vertically, as it could be bent a little. This piece was already cleaned in time as the edges are not sharp on handle and crossguard.
would you think it is sandblasted or wirebrushed recently in the collectible trade or during service/re-furb? It is pretty obvious some layers have been removed, as the Steyr 1886 mark is partilly faint. On the other hand, I find it strange there is no edge-effect after sanding: I'd image if you sand the blade, the crossguard would protect the bottom of the blade so that there would be more thickness in that area