Same reasoning as for your other question.

I say "holding" but as in " 'old on, guvna" or "'old yer 'orses", or " 'ang on a mo", or as in a "hold point", not as in "seizing, grabbing, securing, retaining", etc.

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Note added at 7 hrs (2011-12-19 22:55:33 GMT)
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Whoops, forgot to say: I don't understand why there should be only one such, however. Unless by potence they actually mean jeu de potences, a place where a set of fluxed items suspended from a jig can be placed temporarily should there be a holdup in the bain de zinc (cor, a zinc bath will fetch a lot of money in your local brocante), thereby freeing the crane for other business. Man, I could have made a mint if I'd gone into time-and-motion studies instead of translating ...