The guess is pretty good. The regulator is a balanced valve, with gas pressure on the high side and spring pressure on the low side.
On preset regs the output pressure is fixed, and adjustable regs have an adjustable spring pressure, which gives a variable output pressure.
There are two obvious places to have gauges, on the high side of the reg, which shows the bottle pressure, and on the low side to show the output pressure.
A fairly simple explanation, i hope it helps.