I bought a 12 chord Oscar Schmidt autoharp in 1967. Its best (most complete) keys were Bb and F. There was another model that had a lot of diminished chords - I never figured out who that was aimed at. There was an Appalachian model with chords that sounded more like what a folkie would want, but it was too expensive for me at the time. There were 15 chord models, but not 21, as far as I know, at the time. In the early 1970s I sold my 12 chord model and got a used 15 chord Sekova ChromAharp. I ordered some extra chord bars from Oscar Schmidt to customize it to keys I wanted to play. At the time I think chord bars they carried cost $2 and ones that were not on any of their models (Em?) cost around $5. I still have that one, but don't play it. I have seen some lovely handmade ones recently.