It's hard to tell from a serial number and in the mid eighties, both were available. They seem to all generically get called "109's" when, as AWAP points out, only the Adjustables are properly called "109's" as the Balanced Adjustables are 156's.
Due to the assist from the pressure on the other side of the balanced poppet, the BA uses a much lighter spring and tends to engrave the seat a lot less over time - unless of course you use the little blue purge button depressor (or a quarter wedged betwene the rubber and metal purge covers) during storage, in which case the seat is lifted off the orifice anyway and spring pressure makes no difference.
If properly adjusted, there really is not much discernable performance difference between a 109 or 156. But regs like the MK 5 and MK 10 are not totally balanced as the piston stem is the same diamater as the knife edge, so the area of the knife edge itself is actually not balanced, which causes a 4-6 psi drop in pressure as the tank empties. So theoretically 156 would need adjusting slightly less than a 109 to maintain peak breathing performance.
Personally, I like to upgrade mine to Balanced Adjustable status as they then use the current S-wing poppet as used in the S600, G250HP etc and offer performance comparable to either. The only parts needed for the update are a G250/S600 annual service kit (the older ones included a balance chamber and most dealers will have a bunch sitting around) and a spring with a dealer cost of $1.00.
Whether you decide to upgrade it or not depends on what your LDS is charging to do it. Annual service parts will run $15.00 for the annual service kit and no more labor is required for the coversion than for an annual service ($25.00 per stage at most) so the additional cost in nearly all cases is $1.00 for the spring. But...I have seen LDS's charge as much a2 $75 to upgrade a 109 to 156 status and that is basically a rip off.
