I was on a dive boat today and noticed one of the regulator first stages resembled your mystery first stage.....at least the body did-not the cap. I had a look at it and the yoke was stamped "TUSA." The second stage was also branded TUSA. I didn't think it proper to ask if I could take her regulator apart, but that might be a lead worth investigating.
Good luck,
Couv
Good luck,
Couv