astrosa, you're quite correct on the history of the 3 Bro's cars. Keil-Line owned the tooling to the Budd-built cars, but never produced any. After about 1984, Keil-Line effectively didn't produce anything from the line. Advertised 'em fer shur, but never made them again.
John Keil had originally acquired the tooling from Holgate & Reynolds, who last produced the Budd style in the 70's. H&R apparently bought the tooling from Zircon Models. The first reference I ever saw to these very long-lived kits was a Zircon ad from about 1959 or 60. No changes or updates were ever made to these kits until Three Brothers began including the wheel trucks pre-assembled.
To my knowledge, though, Herkimer/OK was never involved in the bilevel cars--that would make soliciting their help a long shot, indeed.
But there's hope! Train Station Products has Budd-built bilevels listed in the Walthers online catalog (732-808 and 732-809), but I have so far been unable to find out if they're just vaporware or not. I'd love to see what they make, because I'd already concluded that I would have to scrap every part except the shells from my 3 Brothers cars and scratchbuild everything to get a decent-looking MILW scoot...
For the record, RTA sent one or two sets of Budd-built cars over to C&NW starting in the early 80's, but they always wore either RTA or Metra placards. With their fixed-direction seats, they were very unpopular with riders accustomed to Pullman's reversible walkover seats.