A better possibility, perhaps, is that no part of the omitted words was to be sung at all, and that the printing of the initial letter was only to suggest what was intended to the possibly insufficiently imaginative purchaser of the sheet.

The song is sung that way today, but that doesn't mean it was necessarily sung that way over 300 years ago.

At any rate, the appearance of dashes would tell the broadside customer immediately that the song was somehow naughty, thus attracting his or her attention.