In looking at the NZicon example my first thoughts are that you are correct in that hook quillons do not have oil clearing holes. I checked the eight Britishicon ones in my collection and none of those have the clearing holes. It is possible that the clearing hole was added later, however, if this was the case whilst back in the factory/armoury the quillon would/should have been removed. I also see an inspection date of 1931 which would explain the clearing hole but the question remains; why wasn't the quillon removed?

I still owe Old Smithy a response regarding the actual physical differences in hook construction but it takes time for physical measurement which I seem to have precious little of at the moment. The hook on the NZ example does seem at first to be of a slightly different shape but this could be an optical illusion.

The bluing though is inconsistent. The blueing to the pommel and crossguard does not match that of the screw bolts. In fact the screw bolts are devoid of the normal machining marks and bluing. The bluing on the ricasso area blends into the fullers (on both sides). In all of my examples the bluing is a defined line that finishes before the fuller starts. I have seen this type of bluing previously on Russianicon reproductions.

Nice pick up Smelly Sam ...