The plastic saddle is an easy to spot give away for a Mk 10 versus a Mk 5 with its chrome plated brass saddle.
However the important functional differences are that the Mk 10 uses a smaller diameter piston, and houses the piston and the bearing surfaces in the main body rather than in a separate swivel cap - as is the case on the Mk 5 as well as the later Mk 15, Mk 20 and Mk 25. The larger piston on the Mk 5 (and Mk 15, 20 and 25) also results in a larger size over all.
The Mk 5 evolved a lot over the years getting a swivel cap with 2, 4 and eventually 5 ports, and it also saw the change from 3/8" to 7/16" HP ports and gained a second HP port. In addition the Mk 5 had three different yoke designs - a 2250 psi yoke, a slightly heavier 3000 psi yoke and a much heavier yoke using a 9/16" yoke screw like the Mk 10 and subsequent Scubapro first stages. The Mk 5 also stayed in production and in the catalog along the Mk 10 for much of the Mk 10's production life.
The Mk 5 and Mk 10 both had non swivel turret versions as well for customers who never trusted or wanted the swivel turret cap. The non swivel turret version of the Mk 5 was the Mk 8 and the non swivel cap version of the Mk 10 was the Mk 9.
In both the Mk 5 and Mk 10 you saw style changes in the second stages attached to them with variations in purge covers diaphragms colors and adjustment knobs, and in the last of the Mk 10s you saw a flat, bead blasted finish on the first and second stage, much like the early Mk 20.
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In the OP's case he has an earlier Mk 10 that has small SPEC holes in the ambient chamber rather than larger holes in a groove designed to accommodate a ring/boot to hold the silicone in.
However, the op has a balanced adjustable second stage that has a later style rubber purge cover and a later G250 style adjustment knob.
The early rubber purge covers had round holes and a very small round lip around the chrome plated brass "S" insert in the center. The next style (the first to appear with the Mk 10) used a raised lip that stuck up about 5/16" around the S and provided more positive purging of the reg. The final style is the one pictured with no lip and oval holes.
The original adjustment knobs on the Adjustable were knurled brass and were solid inside the case, These were followed by knurled brass knobs that will milled in the center to thin the stem inside the case and reduce weight and these were in use when the Balanced Adjustable was introduced. The G250 brought the gear shaped adjustment knobs (like the one in the OP's picture) which are molded plastic over a chrome plated brass stem, and there was a final chrome plated brass gear shaped knob used on the late production Balanced Adjustable as well as the much more recent G250V.
In the case of the OPs reg you'd have to look at the serial number to determine if it is an older second stage sold with its original Mk 10 but with some later replacement parts, or a later second stage paired with an older Mk 10 sometime well after the two were sold. Not that it makes any functional difference. All of them can be updated to the same status and all of them can use the current S-wing poppet, orifice, etc.
