For what it's worth to other readers, you can find the photos of the 650B Joe McElroy tandem the OP mentioned at the following link:
http://www.flickr.com/photos/8379107...th/6209278381/

Relative to your question about the location of the seat tube slot & keyhole, this one has the rear seat tube slot facing forward which is unusual for a tandem. However, when you look at the braze-on extensions for the rear rack you can see that if the slot & keyhole were added after the frame was assembled it would have been a lot easier to install the slot & keyhole from the front... so I'm sticking with that assumption on generall placement which has simply become "tradition".

The 650B is also sporting one of those Phil Wood disc brakes on the rear wheel and appears to have a threaded front hub as well. So, I'm also sticking with the notion that the front hub "may" have been spec'd to allow for the use of dual Phil Wood disc brakes. The rim brakes on the 650B are period correct...

That's a very nice example of a boutique quality touring tandem... and the late 70's Santana with it's "classic" decals & Davidson sitting along side it in the photo gallery are also pretty sweet. The Burley's OK, but not nearly as exciting as the Joe McElroy.

FWIW, that appears to be Bob Freeman's Flickr page. Bob is the owner of Elliott Bay Bicycles in Seattle and should be able to answer any and every question you have about tandems that were produced in the Pacific Northwest. Great guy... awsome shop that front's Bill Davidson's frame shop just down the street from Pike's.