The more I look at these,
the more convinced I become that they are an enhanced version of the 1960's model 560.
The bore size (thought the 560 was three-valve top-action) is even the same as the 331 - .665" (which is a telescoping-tubing size down from another bore size Holton used: .750")
In my experience the 1960's (so-called "student") model 560 tubas are quite rare...
As is known (by those who follow the repairs/frankentubas forum, here) I acquired one and have been reconfiguring it to something (albeit with a 5th rotor, which is an FF
half-step length) quite similar (though slightly larger bore size) to the 331.
My project isn't completed, but is completed enough to be playable...and what a player it is (not for sale, It's being built for my own use).
...and (just fwiw) I'm certainly not interested in offering it for sale for $6K...so
HERE (with this Holton model 331 listed for sale) is a bargain, in my estimation...
...oh yeah: and the 331 is a REAL Holton tuba, and NOT some "I wonder who actually made that" tuba.