There was the original Mk 2 in the 60's that led to the Mk 3 in the 70's, the primary difference being a smaller diameter piston in the Mk 3. The Mk 3 itself went though some upgrades during it production life - increase from 2 to 3 LP ports, change in the yoke attachement/first stage body and various yokes rated for higher pressures, and a new cap designed to accomodate the SPEC kit (Silicone Protected Environmental Chamber).
The Mk 3 was followed by the generally similar but short lived Mk 200 and by the re-introduced MK 2/Mk 2 Plus which is essentially a late production Mk 3 with the Mk 2's large diameter pistona TIS kit (Thermal Insulating System) in place of the SPEC kit, and an additional LP port (for a total of 4).
The MK 5 was the original high performance balanced piston workhorse and was one of the most copied regs ever made. The Mk 5 introduced 2, 4 and then 5 LP port swivel turrets during it;s production. The Mk 10 was an evolutionary development of the Mk 5 that incorporated a smaller diameter piston. Both remained in prodcution side by side for several years and the Mk 10 was also widely copied. The Mk 9 was also produced in low numbers and was a Mk 10 without the swivel feature for those divers who felt this represented a failure point.
The Mk 15 was generally similar in design to the Mk 10 but with modifications including more bulk and a bushing system designed to handle higher service pressures up to 4350 psi. It also included a larger diameter piston (like the Mk 5) and a concave seat designed to increase flow rates.
The Mk 10 Plus was a Mk 10 upgraded to use a concave seat and a piston with a rounded seating edge that increased flow rates. This upgrade can be incorporated into older Mk 10's. This saw production side by side with the Mk 20's which used the smae basic rounded edge piston in the middle of it's production.
The Mk 20 was essentially a Mk 15 with some modifications to the first stage body to reduce weight, a rubber trim boot on the end, and a different system of retainig the bushings internally. The Mk 15 used a C-clip that could be hard to reach making the reg a little harder to service. The Mk 20 uses a different seat retainer design and replaced the c-clip bushing retainer with a spring that pretty much falls out on it's own during service.
The Mk 20 went through a few minor detail changes to the ambient ports and piston and introduced the TIS system to replace the SPEC system used though the MK 15 and early Mk 20 production. This makes servicing less messy and avoids problems with cross contamination with cristolube for nitrox use, but is generally less efective in very cold water on a high performance reg like the Mk 15/20/25 where the flow rate is approx 300 scfm.
The Mk 25 is just a Mk 20 with an external adjustment for intermediate pressure. Later Mk 25's have a finned swivel cap and incorporate detail improvements in the TIS system.
The Mk 14 was a stop gap diahragm design the SP sold as customers wanted one. Few people speak highly of it.
The MK 16 is an original design that has basically nothing in common with the Mk 14. The Mk 16 has a flow rate of approx 170 scfm and is a good perfroming and well regarded reg.
The Mk 18 is development of the MK 16 incorporating a swivel turret.
SP is developing the Mk 17 which is also a diaphragm design, but I have not heard anything more on this in about a year.
Both the Mk 2 Plus and Mk 16 perform very well in cold water and see a lot of use in this area for ice diving.
Scubapro had their original 080 High Performance second stage in the 60's and replaced this with the plastic cased R190 in 80's. Both regs have the same basic internal design and performance and use the same orifice and seat.
Similarly, Scubapro's original 109 Adjustable second stage was upgraded with a balanced poppet in the 80's to become the Blanced Adjustable and this basic design was put in a graphite compoasite case in the mid 80's to become the G250. With the exception of the G250's adjustable VIVA flow vane, the two regs are functionally identicial.
The newer version of the G250, the G250 HP uses a plastic air barrel and a different adjustment knob and balance chamber arrangement incorporating a seat saver arranglement that reduces pressure on the seat then the reg is depressurized. Users of the older G250 tend to prefer the older model with it's metal air barrel and metal adjustment knob and the seat saver thing has proved to be more trouble than it was worth and the word last year was that this feature was being discontinued.
The S600 is a G250HP in a smaller case.
The G500 was the predecessor of the S600 and european made versions of the G500 can be upgraded to S600 status. The earlier US made versions are not upgradeable without replacing the case, but then they also breathed better than their Italian counterparts and do not need upgrading as they already breathe as well as or better than the current S600.
The G550 is a small cased non adjustable balanced second stage using the same poppet assembly as the G500, S600 and G250/G250HP.
The R380/R390 is a smaller cased version of the R190 using the R190/High Performance's unbalanced downstream poppet design.
SP made the non adjustable G200 for a short period of time and also the balanced but still non adjustable G200B.
Scubapro developed the Pilot second stage in the 70's and this was a sweet breathing, high performance, but over engineered and very hard to service and adjust design. The result was the Air 1 which utilized a center balanced valve design and was again very easy breathing.
The D300 was generally similar to the Air 1 but reversed the locationof the diaphragm and used a coaxial exhaust valve in the center of the diaphragm which had big advantages in terms of case fault geometry which made the reg very stable even at very low cracking efforts. The D350 and D400 are just evolutionary improvement on the D300 and the final D400 improvements are retrofittable into the D300 and D350.
The X650 uses a shorter version the S-wing poppet currently used in all of SP's balanced regulators (including upgraded Balanced Adjustables) and incorporates an adjustment knob and the general case design of the D400. One other major difference is that the X650 uses a conventional exhaust valve in the conventional location, so it has a lot more in commonwith the S600 than it does with the D400. The angle of the diaphragm and general apperance are the only things it shares with the D400.
