Scubapro subcontracted production of the MK V to Tabata, who eventually marketed a clone through TUSA (Tabata USA) and that particular model remained in production into the late 1990's. At one point I had a TUSA service manual that cross matched part numbers with the Mk V.
The design was also licensed to other companies, including Oceanic, and I am pretty sure the Mk V is the most cloned first stage design in history.
The move away from metal second stages had everything to do with production costs. At the time that Plastic second stages started showing up in great numbers (late 1980's) the manufacturers promoted light weight and potentially better flow paths and the ability to add features like adjustable flow vanes, etc, but in truth chrome plated brass second stages are labor intensive and require skilled labor to boot, on top of the cost of stamping the parts, brazing the parts and then chroming the parts.
In comparison you can injection mold a plastic second stage for a couple dollars - and even today the dealer cost on most plastic second stage cases is around $5-6.
Reliability wise, I see all kinds of SP and USD metal cased second stages from the 1960's and 70's that still look very good and function like new - in the case of updated SP 109's, better than new. It is true that chrome plated brass can corrode if not properly rinsed, but many plastics do not age well and have a tendency to crack. They are also much less impact resistant and are much less likely to last 20 plus years in service as a metal cased second stage. That was also I think part fo the reason for the switch. For example the Scubapro Balanced Adjustable was virtually a pinical in conventional metal cased second stage design - it performed extremely well and would last forever. That ran counter to the designed obsolecence philosophy developing at the time and cut into potential repeat sales as well as parts sales.
Even today some SP reps get cranky with SP dealers who continue to service Mk V and Mk X Adjustables and Balanced Adjustables rather than try to sell their customers new regs. Plastic helps solve that as even it it does hodl up in service, it scratches easily and tends to look bad after a short period of time and that creates motivation in some divers to buy a new reg. Of course even that philosophy sometimes does not work. For example SP recently brought out the G250V in part because the "old" metal air barrelled G250 they had discontinued in the 1990's was in very high demand and many divers preferred a used G250 to the plastic airbarrelled replacements such as the G250HP and S600. So in effect, customer demand pushed a company back in the direction of a better and more expensive to produce metal airbarrel and an older, simpler and better but more expensive to produce metal adjustment knob.
I think the odds are good that we may still see some companies moving back to metal cases as well, although I don't think the manufacturing processes will be the same.
