Green’s Dictionary of Slang

harper n.

a penny.

1621		Jonson Gypsies Metamorphosed 35: Two pence I had to spend [...] beside the Harper that was gatherd amongst vs to pay the piper.
c.1650		Le Strange Merry Passages and Jeasts No. 396 112: There was a good merry fellow [...] somewhat doubled about the backe; and his Comrades usually calld him their 9d and their Harper, because commonly 9 pence are a little Buckled [...] least they passe [...] for a 6d or a Shilling.
1937		Nott. Eve. Post 30 Apr. 6/3: It [i.e. 1d] is [...] a ‘harper’ because of the brass penny with a harp on it which circulated in Ireland the reign of Queen Elizabeth.