Flashes of brilliance there, excellent workmanship, with some annoyances (for me) that you might not mind so much, like the too-short rear toptube. I prefer more fork rake (lower trail) on a tandem, to reduce lean-steer. This bike will wander more than I like if the stoker doesn't sit real still, also a bit with each pedal stroke, but you can adapt and might not notice it all after you're used to it. Don't try adding rake to these blades though, t'll ruin that weird/cool reinforcement. (probably obvious, sorry.)
Those dropouts are unique, vaguely ring a bell, but nothing's bubbling up to the top of my memory...
If you're going to ride it a lot I'd consider trying to find or make a larger pulley for the rear brake. "Wire rope" passing over a pulley
will fatigue and eventually fray/break if the pulley diameter is less than about 50 times the cable diameter. This is the advice from aircraft, from back when controls were done with cables and pulleys. 50 times is a pretty strict standard that no one would do on a bike, but still I would try for something a good bit larger than what's there. On the other hand, it's a very visible spot, so you will (hopefully) notice if the cable starts to fray. They don't usually break all at once without warning. Plus it's "only" a rear brake.
The Phil disk is known to fail catastrophically under hard braking (the worst possible time), but again it's "only" a rear, and a drag brake, not the one you're counting on to stop you. So I'll allow it!
I know, you must be so relieved to get my blessing... not! Just clean up the damn electrical tape please, it's hurting my eyes. Since the Phil was only on the market a few years, that could help date the frame if we think it's original. I'm not sure but I would say '81 to '85 maybe? I was a custom tandem builder then, and we did use them, I even made a custom tandem for Mr. Phil Wood himself that had no canti posts, Phil disks only — against my recommendation!
The cap'n seatpost is the original (French) LaPrade, the one that SR licensed and made ten million cheap copies of. The original is very light and cool, take good care of it.
Do you have any Eclipse panniers? That rack has extruded channel shapes in the sides, that the Eclipse panniers slide into, a pretty neat method. Modern panniers have locking hooks that work as well or better, but back then most competing designs had plain spring-loaded hooks that could pop off if you hit a pot-hole or cattle-crossing. Sometimes causing damage when they unhooked. So I was a huge fan of the Eclipse method. The panniers come up on ebay now and then. Oh and I might have some spares I'd let go, contact me by PM if you want me to look.
Mark B