Tom,
I still believe the cars are Weaver cars. It is not uncommon that when the tooling was acquired by a different manufacturer that the original manufacturer's name would remain on the tooling for a while before the tooling was updated or modified. It was very common in HO with Penn Line still showing on the inside of Bowser HO kits after they acquired most of Penn Line's tooling. The same is true with Varney when Bowser acquired some of their tooling as well. My dad was getting parts from Bowser in the 80's for restoration of Varney steam locomotives and many of the parts had the Varney stamp even then. I would suspect that similar things happened with the AMT to Kusan / KMT to Kris Model Trains to K-line and Williams tooling.
The only reason I'm insistent on them being Weavers is that Williams never marketed a non-domed set of cars, but Weaver did make these sets. If by some chance the set you have is indeed a Williams it would be a very rare set. If it weren't packed at the moment, I'd be curious to see what is marked on the trucks of my Weaver car from that era.
This is part of what makes this hobby so great - the mystery!
