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A-line made the original kits, then Intermountain started offering them as RTR models in partnership with A-line. I don't know what the current status of that relationship is. So far, this is the only model of the Gunderson Twin Stack in HO scale that I know of.
Ryan H
I have one set from intermountain and another made from the A-line kit which I never finished detailing.
Come to think of it I have another A-line kit I’ll likely never build somewhere…