Fundamentally, scuba regs haven't changed in decades. Modern first stages are generally 1 of 3 types; 1) Unbalanced Piston (also known as flow by piston). Super simple, but intermediate pressure drops as tank pressure drops (hence unbalanced). The Mk2 is probably the most common/well known of this type. 2) Balanced piston (flow through piston). Nearly as simple as the Unbalanced piston, but IP remains stable through tank pressure ranges. The Mk5 is the earliest I'm aware of, and the Mk25 is the current descendant of it. 3) Balanced Diaphragm (usually just called a Diaphragm). Higher parts count, but balanced (stable IP) and all moving parts are inherently sealed. The Conshelf series is one of the earliest, but it's actually the same internally as the Royal Aquamaster double hose's first stage.
Current production examples of the 3 types include; 1) The Mk2, still being produced. 2) Mk25, descendant of the Mk5. 3) Aqualung Titan/Micron/Core. Descendants of the Conshelf/RAM.
Modern second stages are generally of 3 types; 1) Basic Downstream. Simple, reliable, but unbalanced (work of breathing will be affect by IP... lower IP will make it breath harder). SP 108 and Aqualung Conshelfs are the common/well known old standbys for this type. 2) Unbalanced Adjustable. Slightly more complicated, still unbalanced, but has a user adjustment knob that changes cracking pressure (work of breathing). SP 109 is the common early version of this style. 3) Balanced Adjustable. More complicated, but is balanced (breathes nearly the same regardless of IP) and has a user adjustment knob. SP 156/BA is the early version.
There are some other variants, but these are the common stuff.
If you want to run with the Mk2/108 set, you can find rebuild kits here:
Store | Vintage Double Hose ,
Store | Vintage Double Hose
And look at the bottom of the page there and you'll see a Manuals and Catalogs link... in there you can find schematics and service manuals.
Mk2 Teardown/Rebuild:
I didn't see a video on the Tube for a 108, but VDH did a really good one on the Conshelf, which is close enough to use as a reference:
Let me know if I can help,
James