'When twa sit and one stand' is fairly obvious. Please don't expect exact matches in these analogies. The two stones are at once the man's testicles and the two mill stones. One stand is of course the penis and perhaps the miller who stands to do his work, or perhaps the spindle mentioned in a later verse.

The rest is pretty explicit but I'll have a go later.

'Peggy not to learn' one can only guess at, unless it's a misprint or mishearing. 'Peggy not forlorn' rhymes better with 'corn'.

The song is repeated in Farmer 2 p151 where it is dated at c1707 but Chappell in 'Popular Music' gives a long discourse on the tune which is quite a lot older being used for the earlier song 'London is a fine town' much used and parodied. See pp218-220.

BTW in Pills it's Vol 5 p139. I had bother finding it because it has only the title 'A Song' and I had given it the editorial title 'Bonny Peggy Ramsay' Chappell calls it 'Peg A Ramsay, or Peggie Ramsay'.

More anon.