Lester,
This isn't a reworked round spike bayonet into a cruciform, it is a SINGER cruciform bayonet reworked and remarked. Dimensions on this bayonet are correct for a cruciform, it is an original cruciform. I have no idea why they would decide to rework them but someone did. Possibly because they had old stocks that were dirty/damaged/rusty and they decided to rework them for export rather than toss them out.
As for the marking, I was just taking photos of some of my other bayonets and discovered my No9 Enfield bayonet has the same type mark only it is 98(broad arrow)C so it is evidently a correct Enfield inspectors mark.