It is rather odd that so much has been released about every other aspect of Stairway To Heaven, but the source of those mysterious recorder parts remains a secret. Yes, Tull and Zep both recorded their 4th albums concurrently at Island Studio's in London. Page hardly mentions that part, concentrating on the lyric part later at Headley Grange. Robert Plant came up with the lyrics after the main recording was completed at Island. JPJ was listening to Plants first verse, picked up his bass recorder and laid down the descending bass recorder line we hear today. That is the extent of what Zep has said publicly about it. Jethro Tull was signed with Chrysalis Records in 1970 and Led Zep was signed with Atlantic. Any official mixing of labels would have required an army of lawyers. Stairway is one of the biggest songs in history and is an entire business unto itself. Uncredited guest players would lead to all manner of unwanted exposures and liabilities. Tull was clearly using recorder ensembles multiple times on Aqualung. Anderson had never played a recorder before on record. Led Zeppelin had never used recorders on record before Personnel from both bands did meet and rub elbows during their time at Island. In interviews, Martin Barre has said he and Jimmy Page did discuss guitar soloing and Page was in the control room when Barre recorded his solo on Aqualung. If there was some insest with Jethro Tull members playing their recorders in a Led Zeppelin session, no one would ever admit to it and the entire situation would be treated exactly how it is being treated now. But you have to observe, if you listen to the recorder ensemble work on Stairway and compare it with the recorder ensembles on Aqualung, it sure sounds like the same people playing the same instruments.