Lowboy, I have been examining ads and such for the Hohner button melodicas, and I’m pretty sure it is the HM-900 soprano that appears in the earliest ads, for example one I have here from 1960. I’m unsure as to how many different button models there are, as I see some photos in different colors, but the most prominent are the green HM-900 soprano, the red HM-901 alto, and the grey Student HM-600, which has 20 keys. The HM-900 appears to have started production late in 1958, with the other two in production by 1962. These seem to have continued in production at least until the early 1970s. Hohner’s most aggressive melodica advertising ceased around that time, which makes it harder to track later production years. Those three models were followed up by Piano models, which included the Student HM-920, which seems to be pretty rare.