Unusual keys are not necessarily unusual if there is a brass band connection, as with Salvation Army. Most brass band instruments are pitched in Bflat, eg. cornets, baritones, euphonium,
which means they look at a written C, but actually play a Bflat. A few instruments are. pitched in Eflat, tenor horns, and some bases. They look at a written C , and play Eflat. (Forget trombones, they're a slippery lot!)
It figures therefore that if playing from written brass band arrangements, which would have been quite common, you need some concertinas sounding Bflat and some in Eflat,
Les Branchett