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Can you guys tell if this is a true double or just a five valve F?

Depends on what you want to call a "double". "Compensator" is more like it. Consider the plumbing from the back side of the horn:



With the change valve in the straight-through position, the horn functions as a 4-valve F, with the added burden of having to pass through 1 additional section (that of the change valve). The exit of the F plumbing heads for the bell.

With the change valve in the CC position, the path first goes through the "CC" side, which is really nothing more than another 4 valve set (the rear sections) attached to tuning slides that represent the difference between F and CC branches for a given valve. The path then goes back up to the change valve and goes through the F plumbing and thence to the bell. So, you're passing through no fewer than 10 valve sections when you're in the CC position.

I'd expect that this horn would play pretty well in the F position, but be kind of awful in the CC position for any but the low register notes. In other words, don't expect to use it as a CC throughout its range. All in all, I'd rather have a 6-valve F.