Really nice photos of the K5. I am gong to light off the big computer and see if I can post a K5. The one I did had aluminum superstructures, and cast iron main frame and drivers. Elegant, when finished.
I believe these two were the major suppliers of 17/64 scale. I note that both call it "17/64 Scale, O Gauge". That is the way I have always heard it described.
Lionel did the UP Streamliner in 17/64, and about half the Adams Diesels are 17/64. I believe the Williams PRR steam switchers were also 17/64.