The world is indeed very small, Quimby...
Jack Chappell is the individual that I interviewed while working for Rodale's. He "told" me that he was the chief designer, but I later heard from reputable sources that he often would inflate his stories a wee bit. As was common with any design, the MR-12 was not revolutionary, but evolutionary. I have a Dolphin II which also used the balanced diaphragm design. It is very similar to that of the Waterlung Navy model. Anyway, it was very overly complicated, especially the HP balancing system along with the diaphragm assembly.
One of the problems with early single hose models was that the exhaust valves were so tiny in relationship to the internal case volume. The exhaust tees were also small and narrow. This wrecked havoc with the exhalation effort.
The Titan first stage was actually nothing more than the housing for the Trieste double hose reg. In fact, if you pull the cover off of the Titan, you can see the blanking plug used to seal off the port used for the Trieste. Voit even pushed the modular concept on this for a time, as a sort of conversion unit.
I also have found that Voit switched over the IP valve assembly while bringing in the MR-12 name plate. The pin assembly that pushes open the valve became considerably shorter in the new units. These were not interchangable as I found once when putting a "long" pin in a short housing. When I charged the first stage, the pin was permanently bent by the IP pressure. What is strange is that the cut off on this change made no real rhyme or reason. As some later units use the earlier design. This was probably due to parts being left over.
Another evolutionary change was that of using a thinner second stage diaphragm that was more flexible. I have even gone as far as switching these to a later silicone type, and that reduced the breathing effort a noticable amount.
So.....as far as your dates go, I would say they are accurate. The switch over to the MR-12 name was one that appears to have covered at least 500 specimens or so. I am guessing that the Voit reps pushed many of these onto the more presitigious shops across the country.
My early version marked MR-12 uses a first stage that is marked "patent pending". It also has more squared off edges rather than the common more rounded contours. It also lacks any LP or HP stampings. The yoke sides are also noticeably thinner.
The Swimaster label was used for dive shops, while the Voit labeled versions where sent to the sporting goods stores. That way they could market to both sides. Interestingly, the Voit models used the cheaper to manufacture metallic labels rather than the fancy painted die cast pieces. Little did they know that the Voit varieties would wear so much better. I have a few Voit regs with the unbalanced piston fist stages that are still in the factory boxes. I also picked up a beautiful Voit Viking II that was the equivalent of the MR-12. It is probably my favorite reg.
I sure do like the HP balance chamber that came out with the MR-12 III design. The original had to be replaced due to the O-ring and nylon spacer being crimped into place when the unit was finished. I have tried to figure out a way to replace the O-ring without any luck. If you can think of any viable method, I would be most grateful.
Another prize is one of the first MR-12 II models. This uses the vortex tube. The first stage has 3 LP ports rather than the common 2 that were found on the original. I also have two that use the original first stage. When Rodale's ran the piece, we ran a '73 edition MR-12 along with a MR-12 II with the vortex tube. The tube does make a difference in creating a low pressure area that assists in holding open the valve. The initial cracking effort was about the same, but the tube and gas deflector made a noticeable improvement. The tilt-valve Little Gem reg was terrible. No wonder why nobody wanted to use one below 40' or so. I refer to their upstream valve design as a "cork in the bottle".
You might also find it interesting to note that when the MR-12 III was rejected by the latest Navy tests the engineers simply changed the exhaust valve to a more supple silicone and enlarged it by more than 30% in area. These two changes allowed the next edition, the Voltrex, to pass with flying colors.
My reasoning for recommending not using vintage regs is purely from a liability issue. I dive with mine under controlled conditons, but I certainly can't advise others to do so.
Yes, my respect for the MR-12 goes very, very deep. I use the latest Mares ones for all of my diving. From exploring wrecks to the springs of your home state, you can always find me using them. BTW, the HP valves from the newest models are the ones I use in the vintage ones. When you find a great design, you stick with it. Like I said, the MR-12 is evolutionary, not revolutionary.
Quimby, thanks so much for adding to my knowledge of the MR-12. I certainly am not am expert, but I love to hear from those with the early first hand experience. I began diving in '75, but used almost all Voit gear. I still have almost all of it. A close friend and I make several vintage dives a year. For those who haven't had the pleasure it is a pleasant touch with yesterday. There are many times while gearing into doubles, dry suits, and deco bottles that I think of days when it was much simpler.
I certainly hope that you can share more of your knowledge with us. Veterans such as yourself are too far and few.
Best Wishes,
Greg