I don't think you want this funky version of the Arai brake, this is the one that was more commonly used on tandems:

Arai Drum Brake Tandem Touring Bicycle Bike Drag | eBay

And really, rather than getting a used Arai why not get the Maddock brake? Mad Dog Drag Brake (incorrectly called a "Maddox" brake on the Tandems East website)

About the same price but much lighter. And they're really beautifully made.

The Arai brake wasn't intended to be a bicycle brake, so was weight wasn't a consideration. The Maddock just seems like a way better choice.