It's a sighting device for helping set accurate 45 degree and vertical lines (pitch and yaw). You align an edge of the device with the horizon. Though flying 'perfect' lines (as viewed by judges from the ground) is a little more complicated than simply setting an edge on the horizon and expecting the judges to see it as a perfect line.
Here's close and cozy. The white-haired pilot was Ken Cianchette, a real teddy bear of a guy who'd stop and talk to any kid or adult who wanted to get close to his baby. His company, Cianbro Corporation(short for Cianchette Brothers) built many highway bridges around the United States. Cianbro is now a giant in the Wind-farm business.
Ages ago I was a partner in a Pitts S2A based in Tacoma, WA (N526DP). I think that plane is somewhere in the NE now. I hadn't seen an Aresti diagram since I competed in Intermediate IAC contests way back then. This brought a flood of good memories back. The last time I felt this way it got very expensive (buying in to the Pitts). I hope I can enjoy watching your video clips without having to buy another plane!