The speculation sounds feasible. There weren't many records from Yamaha regarding production in these days. I had recently emailed Yamaha directly about my Vito YAS-21, which shares a few traits with other Yamaha models. It took them some time to look into it, but stated that Vito Yamahas were going through a lot of "running changes" at that time. If Vito just gave Yamaha a contract, it would make sense that Vito would want to use the rest of their existing parts inventory. From what I've seen, Yamaha also wasn't afraid to experiment at this time. They were probably learning a ton by manufacturing parts to work with other parts that they had nothing to do with. All in all, this is just an educated guess, and that's about the best you'll get from Yamaha themselves on the matter.I do not believe this is a Conn-made horn or associated with Conn.
OP is correct, I believe. It's a Vito Kenosha body (OR partly a Kenosha body, such as bellpiece for example) which Yamaha then outfitted with its keywork. There are also Altos which show up w/this sorta hybridization of Kenosha-Yama elements.
The table, for example...is Vito-Kenosha, it is not Conn....because Conn never omitted one of the rollers as Vito Kenosha did. Note the tube to bow connection is screw-tension ring, a Yama detail. While the table is Kenosha, note the stack and side keys are pure Yamaha mechanism. Note the neck profile is not Kenosha-esque, but Yama esque simply with a heftier brace than your typical Yama 21.
I would say this is a Yama tube and perhaps bow attached to a Kenosha bellpiece. As the bellpiece was LH bellkey, Yama used the Kenosha bellkey mechanisms for that part of the horn.
The speculation being these were fabricated right when Vitos switched from being Kenosha-made to being produced by Yama. Body parts, some keywork, and keyguards, etc. from Kenosha were outfitted with keywork and other details made by Yama, before a full 'switch over' to total Japanese fabrication. When they ran outta the Kenosha parts, or when Yama had tooled ans started producing their own bellpieces for this model - the bellkeys moved to the right and the table changed... everything was full-on Japanese-made.