Way Back in 1963, Musser. Prices and a letter
There is a vibraphone thread here...
Last summer I acquired a 4.3 Octave Musser Marimba from my aunt she bought in 1963. She saved all the papers and one of them was a catalog that had the price for both the marimba and a Musser Century which were $925, $1150 respectively (pic of catalog page attached). Since the M-55 is not in the catalog, I assume it was made sometime after that.