The machine you have is a Rowe-Ami model "MM-1" a.k.a. the "Music Merchant of 1967". You want the red selector keyboard pushbutton for it. Go to
www.victoryglass.com and you can order all the parts needed from them. It's best to call them if you don't know/have part numbers handy.
Here's there part numbers for the manuals:
70-3312 MM1 Service manual 186 pages $39.50
70-3313 MM1 Parts Catalog 84 pages $29.50
70-3314 MM1 special offer both manuals above $59.95
The manuals they sell are NOT cheap photocopies, but rather are very high quality offset printings from new printing plates. They are very detailed and worth every penny IMO.