G'day again Jim,

With a set of six 'stops' (each of which controls one bank of reeds) this sounds like one of the big 'windjammers' (melodeons) that were popular with the German immigrants to Australia in the 1880s. most of the reed sets will be either different octaves of the diatonic key (C ... but "old high pitch")- or slight variations to create a "tremolo" effect.

However your fourth stop, which you say "... plays a different scale - Minor I think? ..." may actually be a "Quint" set ... more or less a harmony set (mostly in fifths ... thus 'quint') meant to be selected along with one, or more, of the nominal key sets - to create an extended harmony. Those few I have met that have encountered such reed sets ... haven't been too enthusiastic - but we don't come from quite the same musical history!

I do have a photo, somewhere, of Mark Schuster - performer, collector and folklorist, from Toowoomba in south-east Queensland - playing a big 6-stopper at the (Australian) National Folk Festival some years back ... and a CD of him playing his traditional music on similar accordions.

Regards,

Bob