I really didn't expect that. Backing musicians don't usually get songwriting credits no matter how much their contribution adds to the success of a record. It could open the way for a whole lot of similar cases. The silly thing is that some of the other parts were played by session musicians. I recall the drummer being paid 15 guineas for the session while the group's drummer Bobby Harrison, who did not play on the recording, picked up his share of the performing royalties & presumably still does. http://www.procolharum.com/eyden2.htm

This was not unusual in the recording industry in the 1960s. You might be surprised to find out who was actually playing on those old hit records.