The Mk V ws made from the late 60's through the mid 80's although there were several changes and a 60's vintage Mk V was a lot different than an 80's vintage Mk V.
Early Mk V's had a single 3/8" HP port. SP went to two 7/16" HP ports on later Mk V's.
Early Mk V's had a two LP port turret and in the 70's SP adopted a new 4 LP port turret. In the late 70's, also went to another new 5 LP port turret and made changes to the swivel cap covering the ambient pressure chamber incorporating much smaller 1/16" holes to accomodate the SPEC system (Silicone Protected Environmental Chamber) that greatly decreased the risk of freezing up in cold water.
As indicated above the original Mk V yokes were designed for 2250 psi service and this was updated to a slightly heavier 3000 psi yoke that still used the small diameter yoke screw. In the late 70's and beyond a heavy yoke with large yoke screw was used.
It can be hard to tell what was made when as many early Mk V's were upgraded in the field. But a MK V with one 3/8" HP port is definitely an early Mk V even if it has a 4 or 5 port turret and/or a SPEC swivel cap.
Both the MK V and Mk 10 were in production along side each other for several years. The MK V had a cult following and many divers preferred it to the smaller Mk 10. It had some advantages as the piston head diameter was larger which made the intermediate pressure a few psi more stable on the less than perfecrly balanced straight shaft MK 5 and Mk 10 piston designs. The large head also potentially made the MK V more repsonsive although the Mk 10's smaller headed piston had a larger bore diameter through the shaft and fewer sharp angles and, consequently, had a higher flow rate at low tank pressures.
SP reverted to a larger first stage body and larger headed piston similar to that used on the Mk 5 when it designed the Mk 15, Mk 20 and MK 25 and that I think says a lot.
The "109" second stage is called the "Adjustable" although many divers call both Adjustables and Balanced Adjustables "109" 's. Externally they look the same but the Balanced Adjustable has a balanced poppet and is considered to be a 156 in the latest revision.
Originally, after the introduction of the Balanced Adjustable in the early 80's) SP sold an upgrade kit that included a new purge cover with "Balanced Adjustable" printed on it. These are no longer available but an "Adjustable" can be easily upgraded to a "Balanced Adjustable" by using the current G250/S600 annual service kit as well as a G250 balance chamber and spring. (The balance chamber used to come with the annual service kit but is no longer included.) Very early "Adjustables" (late 60's and early 70's) may require a new lever to prevent the lever from binding on the poppet. Springs and balance chambers are about $1.00 each and a lever runs about $4.00. But I have seen some dealers charge as much as $75.00 for the upgrade.
Like the Mk V and MK 10, both the Adjustable and Balanced Adjustable were in production along side each other for a few years after BA was introduced.
Three different rubber purge covers were used with the older ones having round holes and a small rounded lip around the purge button. 1980's era Adjustables and Balanced Adjustables had a cover with round holes but a higher raised lip around the purge button about 1/4 inch high. Late production Balanced Adjustables used a cover with rectanguar holes and a very low flat topped lip around the purge button that is virtually flush with the metal purge cover. The last ones produced (mid 1990's used a matte chrome finish like the then new MK 15. Consequently, replacement chrome plated parts will come in a matte finish rather than the shiny chrome used on most Adjustables and Balanced Adjustables.
Adjustment knobs also varied with early ones being quite solid with a knurled knob. Mid production knobs were also knurled but have some metla machined away on the intenral end fo the knob to create space for silicone grease to be used and to prevent dirt particles from binding the knob. Late production knobs used a splined adjustment knob in both chrome metal and finally plastic G250 style versions. The G250 and Balanced Adjustable were identical internally with the excepton of the G250's adjustable flow vane and both were produced conurrently for nearly a decade. Like the Mk V, the Balanced Adjustable had a lot of die hard fans. Personally, I'll pay more for a good BA than I will for an 80's or 90's era G250 as a the BA wil almost always be in better condition.
The "Balanced Adjustable" uses the same poppet as the S600 and G250HP and breathing performance is essentially identical. So unless you are a slave to style, or truly believe in the light weight second stage/jaw fatigue myth, you don't gain anything trading in a BA for an S600.
