This phrase is probably unfamiliar to non-Brits.

Here's the entry from Eric Partridge & Paul Beale, "A Dictionary of Catch Phrases" (2nd edition – London: Routledge, 1986):

Keep your hand on your ha'penny (till the right man turns up) is 'an old piece of advice to an unmarried girl' (Petch, 1969): c. 1880-1970, but little used since c. 1960. Lit. 'Keep your hand over your genitals until...' The orig. is of linguistic interest, for Grose, 1785, has money, as in a domestic nurse's admonition to a female child, 'You're showing your money', in the sense 'vulva'. There is a var., three penny bit; and the expression acquired a secondary meaning: 'Be careful or you'll find yourself expensively involved': C20; by 1970, ob. (Skehan, 1977.)

The phrase occurs in H.E. Bates' novel, "The Darling Buds of May", 1958:

'See you soon!' she waved. 'Any time,' Pop said. He laughed merrily. 'Don't do anything I wouldn't do. Keep your hand on your ha'penny ...'