This does not look like any layout from what we call an "accordion". It looks like it may be from a diatonic instrument, which I am not familiar with (and many other people on here are not either). You may have better luck asking on melodeon.net which discusses diatonic (non-chromatic) instruments.
I believe you need to be a melodeon.net member to ask a question and I read they were not accepting new members.
Have you tried Buttonbox or LIberty Bellows or Bellinger's Button Boxes on your side of the pond?
I believe this layout is not for a 4th apart (D/G, or G/C) diatonic.
It looks useful for Irish type layouts - 2 rows a semitone apart - which come in many versions: B/C, D/D# etc, and are by their nature chromatic.
Info: Old semi-tone boxes had a LH layout that was not very useful - didn't much matter at the time as LH little used - not so true now.
: I believe apart from UK, 'melodeon' is reserved for one-row boxes, elsewhere 'your' box is often a 'button accordion.' The original (Demian 1829 Vienna ) accordion was an accordion because it only played chords.
It isn't such a familiar article that children grow up referring to it by name, so you'd probably get different stories, but in my limited sphere on the North American west coast I hear "melodeon" for those diatonic instruments that are not concertinas.
Hi - thank you all for your replies. yes, this is for diatonic accordian. it seems that many diatonics 'mix' their buttons, eg: C on push and G on pull. it just seems more logical to put C's with C's, G's with G's, etc. But i am a beginner so perhaps i am missing something. While i do play trad irish on my B/C, i am working on non-irish waltzes, mazurkas, etc.
There is a very logical reason that the chords are "mixed" as you say. Remember that the diatonic button box developed with traditional music in mind. This music makes a lot of use of contrasting a series of notes in a fundamental chord with one in the fifth and/or fourth chord. The "mixed" bass chords and the corresponding bisonoric layout of the treble keys allows the player to more easily accomplish this type of music, while using the additional rythmic accents of the opening and closing of the bellows.
This layout is also convenient because, although nowadays the piano and chromatic button accordion allow playing in all keys in all direction, the "mixed" layout allows the instrument to be smaller, lighter and more affordable, although more limited in sonic variability.
PS. Where I live in the US, I have never heard anyone use the word "melodeon." Average people call diatonic button boxes "accordions" or "squeezeboxes." Musicians call all diatonic button boxes "concertinas" whether they use rows (melodeons) or patterns (anglo or chemnitzer concertinas).
Diatonic = Using only the notes in the key Bisonoric = Each button plays a different note on push and pull.
Any 2-row a semitone apart is chromatic – it can play the chromatic scale. Your B/C is a bisonoric chromatic accordion.
The LH layout example allows you to play an ( almost ) octave on the bass notes in one key, runs in the useful keys and simple accompaniment (aka Three Chord Trick) in these keys:
Key of C: G C F [A] Key of D: D G A Key of G: G B D [E] Key of A: A D E Key of E: E A B
If these 'chords' omit thirds, the chord in brackets [ ] would add more interest.
More here: Keyboard Layouts - 2 Row Diatonic Accordions (melodeon.net)
(PS no idea why some of this is in bold)
I keep thinking about this and how to explain it better. In certain types of music that rely on the button box (cajun, zydeco, tex-mex, tarantella, Irish, others) the rhythm is so important, as these styles are often dance music. The fact that you can change from C to G (for example) without moving your finger, and getting the percussive rythmic effect of the bellows change is HUGE! In fact, it can be argued that it is 92% of what gives it the drive and style. That's why the basses remain "mixed."