Hey Matt.
Your REG is the last REG to be manufactured by US Divers if in fact it is a 1973 model and is a great one to rebuild. Welcome and have a blast. As I stated to A.Van Wyck who is also new to double hose I quote/paraphrase the following:
You will find that the DH regulator breathes moister and is far less intrusive when photographing subjects close at hand. (Macro). That and it just looks cool. Start shallow at first (10 feet or so in a swimming pool if available ) and get used to the various difference in clearing, underwater donning and doffing, weight use etc. . Take your time and build up your confidence in your regulator and your techniques.
After initial rebuild and IP adjustment, wait about a week or two and re-check IP (should be right at 135 psi) the same applies to the "Phoenix" if you install one. This is for compression of the new 1st stage main diaphragm and "settling in" of the new components. This will make your experience even more enjoyable.
If you haven't already done so and you are thinking of keeping the regulator in its original configuration, I would think about a "Hookah Port" adapter for a safe second (Octopus) as a fall back for peace of mind until you are ready to go full tilt "Vintage" if you so desire. Also, investing in a "Banjo" fitting for a submersible pressure gauge is a plus if you do not have a functional "J" type tank valve. All this applies only if you keep the regulator in its original configuration as the addition on a "Phoenix" first stage preferably with a new "HPR" second stage would cancel the need for a "Banjo" and "Hookah" adapter. You are lucky in that you already have a model that contains the "Long Yoke" designed for a SPG/Banjo fitting and it is rated at 3000 psi only made in 1973!
Don't be shy about asking questions about rebuild peculiarities and other such stuff. I have found that just going through the back posts on this Forum a great help as well. The folks on this website are EXTREMELY knowledgeable and will go out of their way to help you. Also, see Alec Pierce's videos on Youtube at:
https://www.youtube.com/watch?v=Eq0aHH3fpOg
Alec has been diving for 58+ years and in his series of Vintage Diving equipment he answers a lot of questions that new vintage divers wonder about.
Have fun and again , Welcome to the World of Real Fun Diving!
Russ