The music industry is full of "dirty little secrets." Up until the early 80's there is a chance that we may not being hearing the musicians listed on the label or in the band when we listen to the record. It is really hard to write good music, and many times the band members were delivery boys that were just dragged in off the streets and showed a few licks to join a garage band, just before a label signed them. Even today, studios will keep musicians on hand to record over band members. My cousin is in Muscle shoals and he is the actual drummer on almost all bands that record there. He just got in the Rock and Roll hall of fame a few years ago. These musicians usually don't get any recognition, but he does draw a percentage.