early history of the chromatic?

The chromatic harmonica was introduced in 1910 in a 10-hole model with the same note layout as a diatonic. This was the Hohner Model 260, which eventually changed to the solo tuning now familiar on chromatics. The 260 was discontinued only in 2013 after over a century. The first 12-hole (3 octave) chromatic was called the 260 1/2 but the name was eventually changed to the 270 (Super Chromonica) which is still in production. Sixteen-hole chromatics first appeared in the 1930s AFAIK.

Early chromatics had an outside spring. Nowadays the spting that returns the slide to the out position is a wire spring hidden inside the body of the harmonica. Originally it was a leaf spring mounted to the outside right edge of the comb and curving around to lodge behind the slide button. Some players still prefer the outside sping and at least one customizer will retrofit a chromatic with one.

I don't have a lot on early instructional material (1910-1930) although I do have some materials dating from the 1930s and later. Some of those materials stated that the chromatic harmonica was not meant to be played in all keys; rather, the chromatic notes were just meant to supply the occasional chromatic note that might come up in a song in C or G. Whether this was simply the prejudice of the writer (who might have ben covering for his own limited technique) or whether this is a widely held view is unknown. Chromatics have always been available in several keys, at least in the 12-hole size, so maybe this view was widely held or at least catered to the path-of-least-resistance consumer.

Advertising in the 1930s became centered around famous players like Larry Adler. While ads for the diatonic seemed to have been aimed at young people seeking social inclusion - be one of the gang, be the life of the party - ads I've seen for chromatic  clearly aimed at urban adults and was presented to project a glamorous image.