Green’s Dictionary of Slang

a (female) sexual sophisticate [the citation suggests a woman, but in SE the female would be posture-mistress].

[1827		Pierce Egan’s Life in London 3 June 981/3: The attitudes of Spring were almost those of a posture-master; he shook his head, and danced about like a man hung upon wires].
c.1850		Peeping Tom (London) 6 22/1: All the licentiousness and singular attitudes of the most experienced posture-master [...] stimulate the pallid senses of the young and renovate the decayed faculties of the old.

see under moll n.