Wurlitzer used the internal components from the model 3700 to make the 1050. Since most of the parts were the same they didn't republish a brand new manual for the 1050. They only issued an extra 1050 Supplement for technicians to use alongside the 3700 manual they already had. To save paper?
Both the 3700 manual and the 1050 Supplement are included in this one book for complete service information.