Walter Pardon from Jim's mustrad article

Nine times out of ten I can get an old fashioned ten keyed accordion, German tuned, you can nearly tell what is an old song. Of course that doesn't matter what modern songs there is, the bellows always close when that finish, like that. And you go right back to the beginning of the nineteenth and eighteenth (century), they finish this way, pulled out, look.

How good a discriminator is that? Many of the song tunes, and a huge number of dance tunes, were written down in the early 19th and late 18th century. Are all the Music Hall tunes all 'bellows closed' tunes? The way Pardon put it not definitive because it could be a circular argument but the manuscripts might help us test it.

A box player explained to me about the bellows open thing when giving hints on how to work out the key of a tune from watching players, and it was mentioned early in this discussion.