Considering the subject matter of the ballad it's possible 'hautboy' is here a sexual cliche.

I know in our emails I started to move away from 'coins' to 'plans' but looking again I can't help thinking 'sketches' is something more physical. He has got money from the girls so what else could he be bragging about?

BTW I've checked the Oxford Dictionary and though it has lots of definitions one of which is 'plans' none seem to fit easily here. I still think it's a cant term but Googling doesn't bring it up.