Great find!! I especially like the idea of the removable screws to tie the hot or neutral halves together. Probably made much better contact than the skinny little breakaway tabs on today's recepts.
Speaking of which, as narrow as those tabs are I have serious doubts that they can handle a full 20 (or 15 for that matter) amps of feed-thru current. Maybe when I have time to kill I'll set up a test using an IR thermometer.