Some manufacturers of IEC 60309 plugs offer a "flexible conduit" option which I would think could be used in conjunction with a SWA gland, but I strongly suspect the plug in your picture is a hack job, either not terminating the armor at all or terminating a few strands of it to a terminal.
No worries they're still freely available. I make up 16A to 5, 15 or 15A adapters by screwing a long brass bush from the inside out then a lock ring or washer and a surface mount steel backbox. also needs a M4 nut and bolt.
I did 50 earlier this year for a lighting company.
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