Kind of looks like an original Sovereign, back when they were steel, not aluminum. These old fillet-brazed frames are really pretty! It could also be a Noventa, which used "NivaCrom" tubing. Are there any tubing stickers or a model name on the bike? I see a Columbus sticker on the fork.

From Gear-to-Go Tandems' website:

"Prior to switching to aluminum (in 1993) the Sovereign was built from Columbus "Cyclex" (a slightly fortified cromoly) in 9/6/9 gauges and "oversize" diameters. "Oversize" is shorthand for frame tubing too large for conventional lugs. "9/6/9" is bike-speak for double-butted tubes where the wall thickness is .9mm at both extreme ends and .6mm through the rest of the tube. Most double butted tubesets, including the Ishiwata sets Santana used prior to 1984, are slightly heavier: 10/7/10."

Your rear Arai drum brake can be removed two main ways: 1) If you have a really big socket wrench (as it appears you do), you need to remove the wheel, take off the brake-pad assembly (it's the thing with the actuator arm, and it simply fits inside the drum, usually held on by an axle locknut), and you'll see a giant "nut" cast into the drum. Using your socket and wrench, hold the wheel tightly and remove the drum (from memory, you would untighten it clockwise). 2) You can also buy or make a removal tool that has two "pins" and a handle. The pins fit into the outside cooling holes in the drum, and you remove the drum that way. Do a Web search for "Aria removal tool" and you'll find them for sale. I made my own home version with a 2x4, a couple of long bolts and nuts to anchor them, and a hand drill. The socket method is more elegant, but most people don't have a socket that big laying around. I would imagine it's also less likely to damage the cast-aluminum drum.

BTW, you might want to swap out the Blackburn Mountain rack, which is made for 26" wheels, for one that is designed for 700c/26" wheeled bikes. Not a big deal, but it will give you more clearance between the rear tire and the rack.