Interesting viewing on that dvd, learned some of the songs on one of Physical Graffiti sides were originally leftovers from other album sessions.
I'm surprised you didn't know this Riffy. I think that's one of the factors that makes PG one of LZ's most interesting and dynamic albums, a true testament to how far the boys had come artistically since their earlier days with a variety of songs for any state of mind the listener may be in with hard bluesy rock like In My Time of Dying to the more serene Down by the Seaside and the masterpiece Kashmir.
I think Kashmir and In the Light were Page's babies of the album. He incorporated a lot of new elements he and Plant had picked up from their recent travels to Eastern countries to draw inspiration for their music. They observed a lot of street performers in India and incorporated those elements into Kashmir and obviously In the Light. In LZ '75 Stephen Davis recounts telling Page his thoughts on In the Light and Page got a lot of pleasure from the fans' enjoyment of the sound of these tracks. It was all knew to them but they were at their artistic peak at the time of PG and everything seemed to start to unravel first with Plant's car accident in Crete and the death of Karac Pendragon a little later.
Night Flight, Boogie With Stu and Down by the Seaside were recorded during LZ IV sessions, The Rover and Black Country Woman during HOTH sessions and Houses of the Holy of course around the same time since it bears the name of their album of the same title that came out around that time, Bron-Yr-Aur was recorded during LZ III sessions which is obvious with its overall tone with coincides with LZ III's acoustic vibe.