Even though the Oceanic web site only names and titles pages and reg's by the 2nd stage, the reg set should be described by using both the 1st and 2nd stage reg's, if one is to properly describe the reg set.
I have a little experience with Oceanic reg's, since the first dive shop I worked for was an Oceanic dealer, the operator I currently work for is also an Oceanic dealer and an operator in between got dealer pricing.
I have purchased; CDX-5 / Delta 4, PX3 / Alpha 8 (x2), PX2 / Zeta, FDX-10 DVT / EOS, as well as "left handed" Slimline 2 alternate and Swivel alternate (x2).
Back when the CDX-5 / Delta 4 first came out, instructors were offered a really good "promotional" key man price, which is the main reason I bought one. Switching from a ScubaPro MK14 / R190, which had a 1st stage swivel, to the CDX-5 / Delta 4, which does not, was not a comfort issue because the Delta 4 has a swivel. I actually found the Oceanic set up much better in every way, which is understandable considering their place in each manufacturer's line up (top of the line / new vs bottom of the line / discontinued).
Then Divers Direct had a free shipping blowout sale on PX3 / Delta 8's, so I bought 2. Then, wanting another Oceanic 3-gauge console, I eBay-ed the PX2 / Zeta. Not fully appreciating the best function of the PX3, I sold one of the PX3 / Alpha 8's; should have sold it as a PX2 / Alpha 8.
Oceanic seriously pissed me off when the EOS was released, because they quit selling replacement swivel hoses for the Delta 4 (found one here on SB). I would be interested in getting another "gently used" stock Delta 4 swivel hose, if any long hose Delta 4 divers have one they are not using.
A couple years ago, I "upgraded" to the FDX-10 / EOS. I way prefer the CDX-5 over the FDX-10, because of the less than half cost to me as an instructor and lower profile on tank valve. IF / when I buy 1st stages again, it will probably be used CDX-5's. Oceanic repair and service in Hawaii has been great for me over the last decade.
Interesting to see the Delta 4.2 page at the Oceanic web site; still titled 4.1 on my web browser. If they are offering you the "$90 swivel hose" option I'm wondering if it fits the original Delta 4, and if so why didn't they just go back to calling them Delta 4's?
The PX3 and PX2 1st stages both swivel. The PX3 has an lp port on the bottom, which has many neat uses. Other than that I believe they are identical and I would sell the PX2 for a reasonable price, with or without either the Zeta or Alpha 8.