From Wikipedia: The track stems from an old master reel labelled 'Yardbirds. October 10, 1968' (Led Zeppelin were called the "New Yardbirds" during their first months of existence). The master tape went missing for a number of years and allegedly turned up in a refuse bin outside Olympic Studios, following renovations in 1991.

This is an outtake of sorts in that it isn't very refined. There is a more engineered/produced version that actually is included on the re-issued album "Coda" now, although it wasn't originally included when that album came out. Safe to say there is no auto-tune on the vocals, lol.