Halcyon and Scubapro developed an agreement several years ago where Scuabpro would sell rebranded Halcyon back plates (and wings, but Scubapro made some not so great changes to them) while Halcyon would sell the Mk 17 and Mk 25 first stages and the G250V and R390 second stages with some minor cosmetic changes.
Scubapro designs it's own regulators and holds the patents on them, but farms out the manufacturing of the actual parts to Tabata. That makes sense as Tabata has state of the art manufacturing facilities that would not be economical for Scubapro to acquire and maintain with the just the volume of Scubapro regulators that are produced.
There are in fact just a few companies that actually make scuba regulators. Tibet, Sherwood, Beuchat and Poseidon are three companies that come to mind that actually make their own regulators. Most of the rest of the brands either contract out the manufacture of their own designs, or sell someone else's design under license.
Tabata has made Scubapro regulators for 3 or 4 decades now. They were made by Beuchat in the early years of the company, and the Jet Fin was first made by Beuchat and sold under the Beuchat brand). As part of this arrangement, Tabata has from time to time produced Scubapro designed regulators with some cosmetic changes for marketing as TUSA regulators (TUSA stands for Tabata USA). The Tusa Duo Air is a great example as it was an Air 2 with a TUSA label and some re-stying - the internal parts were identical. The Mk 5 was sold by Tusa for years (and in fact the Mk 5 was sold by a number of companies and was one of the most prevalent first stages on the market in the 1970s and 1989s. More recently TUSA sold a rebadged version of the the Mk 16 R390.
Currently Tusa sells the RS-790 which is a rebadged MK 17, and the RS-700 which is a rebadged S-360. The R-1000 is a rebadged Mk 11, and the S001 is a rebadged R-390. The RS-812S second stage is a rebadged S-600, and the SS-11 is a rebadged C-200.
The arrangement between Halcyon and Scuabpro essentially means that Halcyon has Tabata make Scubapro regulators with some cosmetic alterations to market them as Halcyon regulators.
The H-75P is a Mk 25 (pre EVO) and the H-50D is a pre EVO Mk 17. The Halo second stage is a G-250V and the Aura second Stage is an R-390. The differences are minor cosmetic changes. The internal parts and servicing are identical, and the rest of the parts are fully interchangeable. That's good news also if you want a G250V as you can still get it through Halcyon even though it's been discontinued by Scubapro.