Is the retaining spring in the No5 different to the FN spring. It surely MUST be because whereas the FL blade is double edged and pretty flat (there's probably a name for the shape....) the No5 is single edged and had a channel into which the spring fits and retains the blade.
I mentioned some time ago we used a 'fake' blade from India to test the metallurgy against one of ours and while it wasn't exactly the same, it was so close as to be certainly in the same formula - or as close as you might get within a mixing formula as they called it! Mind you,we weren't in a position to know whether our 'fake' was a real Indian 'fake' or an Indian unmarked RFI.
To my tired old eyes, the bayonet looks like a real No5 that someone has polished down to the bare steel and then neglected.
But a word of warning for anyone that wants to set about drilling the clearance hole. The hole crosses the pommel end of the actual tang of the BLADE part of the bayonet - where the actual tough blade abuts up against the pommel. If you drill down through, half way through, the tip of the drill will break through the pommel and onto a half section of tough blade material which will deflect the drill bit and send it off course as sure as eggs is eggs. A lesson learned at Sterling when converting bought-in bayonets to military/customer spec. If yo do this, bore down to the tang of the blade then use an end mill to bore the half diameter of the tang. When it's through, continue with a drill bit.