Well, I think I have figured this out. It turns out (I didn't know) that the diminished on the accordion is actually the 1,3, and 7, dropping the 5th (in C, the Gb). So, a C diminished is C, Eb, A on the accordion. If you add the F note from the next row over, you get a full F7 - F, A, Eb, C. When you would ever need this, I don't know.

yours,

Peter T.