Ah, you're fitting the cable backwards!
Replacement cables would be supplied with the appropriate tags on both ends, the key end would be attached first, and the cable fed through the grommet, after which the jack plug would be fitted to the free end. Workshops (ant telephone exchanges) had a hand-cranked jig to assist in screwing the jack plug on to the (ready made) cable, after which the crimped-on terminals would be fixed to the plug connections and the plastic cover screwed home.
I'd like one of those jigs, as I have replacement switchboard cables (in Chinese Red only, so far) to restore my Switchboard UC and its severely moth-eaten cables with.
Original British keys used the No.10 plug, which provided adequate finger protection. The Canadian keys had a more exposed 'hot' end, and the set was fitted with a plastic guard to reduce the risk of a 'belt' from the buffer stage HT (250 volts or thereabouts). I can't remember what the American keys used.
'Ham' modifications using commonly available jack plugs are a recipe for disaster if used by the unwary: Post Office style jacks with exposed screws (and the tail end of the brass body) are a hazard, and I hate to think of what the modern "all metal" screened plug would do to the user!
Chris.