I'm also interested in this issue. I think the Japanese just "jerry gigged" the lead pipe on a conventional euph, which is a great idea. I'm mean think about it, it's just a bend, a re soldered joint, with a brace. This is just my assertion.
I recall seeing the all girl corps (sorry can't recall the name) with the shoulder euphs at Wildwood many years ago. Also, as I stated in an earlier post, The Bronx Kingsmen had them, WR's as I recall.
If someone from Japan could enlighten us it would be appreciated. Mean while, I have a 2 valve euph that I may try to re do the lead pipe.
It should be noted that before the contra's came on the seen, it was the, what we now call a "baritone", played the lower register assigments, peddle tones and all.
Interesting thread. stein