Romain Merlin, in his work on the origins of playing cards, mentions a Persian card game which includes an entire suit of Slaves, as opposed to a court card of the same name. D’Allemagne says much the same thing, but of an Indian deck, citing Chatto.
In his book of talismans based on the Key of Solomon, Eliphas Levi also calls the 4 valets by the alternate title of “Slave of x,” x being variously ‘men,’ ‘women,’ ‘love,’ or ‘circles.’ (p. 45 of http://misraim3.free.fr/eliphas_levy/cl ... alomon.pdf )
That some occultists make unsubstantiated claims is unsurprising, but we find this same claim in the 1898 edition of the Nouveau Larousse, under the heading ‘Esclave’ (p. 279, vol. 4). No reference is given. https://archive.org/details/nouveaularoussei04laro
D’Allemagne mentions an early deck in which the Valets are called ‘Ecuyers,’ but this term, no more than Knecht or the Fameio of the Mantegna deck, does not fit the bill. Perhaps someone can shed some light onto the matter?