"I was once told by a D/G box player, when being encouraged to pick up tunes by ear in a session, that finishing with the bellows extended was a strong pointer to E minor (that included E Dorian). Was Walter suggesting something like that?"

I'm sure he was, Jag, and I went into this on an old Mudcat thread to which Jim had posted that piece of information. I also mentioned it above but didn't explain the detail.

Whether that's a reliable marker of antiquity is dubious, since plenty of old songs are in the major, but it's certainly true that Music Hall songs are (to the best of my knowledge) never in the Dorian.