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Just came across this on the net. I don't recognize the horn, but I'm no helicon expert.

I have always considered this helicon a mystery. It doesn't align with any Cerveny helicons or sousaphones I have seen; and It doesn't look like the two rotary valve helicons that I know. Neat horn though...

I should have added that Ian's two guesses were the same as my first thoughts...

I wouldn't expect a Martin of that vintage to have rotors.

I wouldn't expect a Martin of that vintage to have rotors.

Well, I just proved myself wrong, according to this Martin serial number and ID list but I don't see any there with clockspring valves. I may need to look a more, but I'm out of time for now. Interesting.

It's the clock spring valves that make it hard to align with Martin. I know of 6 rotary valve Martins, and none have had clock spring valves. But then, Martin was clearly experimenting with the rotary valves, and maybe the pictured helicon was one of those experiments.
Mike