The Magnus Harmonica Corporation (founded in 1944 ) started producing plastic harmonicas (US patent US2407312A) at the beginning of 1948. Besides chromatic harmonicas, the Magnus corporation also produced 10-hole diatonic and tremolo harmonicas. More information on the different harmonicas as well as a short and a more extensive description of the Magnus corporation history can be found on Pat Missins’s website.
The interesting invention is that the whole harmonica was made out of plastic, including the reeds. Furthermore, the slider mechanism works similar to the one used for the Böhm Chromatic with movable reed plate: in this case, the reed plates and the comb are moved to the left when the button is pushed.
Other harmonica manufacturers like the British Mouthorgan Company (BMO) were striving for some of the Magnus success:
December edition of Harmonica World about the British Mouthorgan Company
Chromatic Harmonica History 