Not really odd, Acorn4. Worldwide, I would suggest that more use is made of flat keys than of sharp keys, and not just "in the brass band era", but today.

Are you sure you have no E buttons? A standard Hohner 24 bass has 3 rows (fundamental, major and minor) of 8, being Eb, Bb, F, C, G, D, A and E.

Perhaps it is the session-useful Bm you are missing???

It would be feasible for an accordion tech to shuffle your reeds around to produce a B row, as I have done many times on smaller accordions. A bit costly though as every reed in the bass will need moving, and a couple of reeds imported.

And once you have the B row, will you be back next week for a further rearrangement to allow for an F# row? You wouldn't be the first!

Far better to upgrade to an 80 or 72 bass, but even a 72 bass could give you a problem if, say, you were playing in E major and needed a C#m chord. Guess what? There isn't one. Only a Db at the other end of the keyboard which you have little chance of getting to and from.

So I would recommend an 80 bass as it has the overlap to allow for this eventuality. The lacking diminished chords can always be fudged by using the right hand to add the notes of the chord you need that are missing.

Good luck with your upgrade.