Andy,
It was common years ago for session players to fill in on recording sessions - although this fact was NOT widely known- sometimes due to time factors- studio time was expensive, and if someone didn't show up, or couldn't master the parts in time, producers called in session guys to get the job done quickly. Some of the session guys used to walk in to a session and ask 'who are we meant to be today?"!!!
It was only in recent years that this situation became known....people started to speak out and "own up" to who actually played on what tunes etc.
The show must go on!