Your pond looks so much like my pond which I am also trying to revive. My pond was built in 1940. I’ve found one of the old underground pipes running up the slope to an exposed “spigot” top of the retaining wall but I don’t know where it terminates on the other end. The only other pipe inside the pond basin is the vertical overflow. You can see the other photos in the Introduction forum.
Do you know when you pond was installed? I wish I could find information on how ponds were built and plumbed back in 1940 so I might understand where to investigate and have an idea of how it worked back then — especially behind the rock retaining wall and underground. Any tips on finding this info? Good luck with yours! I’ll be watching!