It is easy to forget that the idea of regulated clocks was radical, revolutionary and new, if not quite in living memory, at the time those words
written into the 2nd. Galileo had just worked out pendulums in the early 1600s, and pendulum clocks first appeared in the mid 1600s.

A regulated clock, one with a regulator pendulum,
was an amazing piece of equipment and represented
the peak of enlightenment.

Even today, right now a google search for "regulated clock kit" shows the reverence for
well-regulated mechanisms.

As a fan of mechanical clocks and owner of many of the same, to "regulate" a clock means to adjust the pendulum (period (commonly by length) and beat) such that it keeps time correctly, and is thus well-regulated.

The term regulated is still used in many areas to mean "adjusted or works correctly". It is willful pretense to state the meaning as "under government regulation" and the people doing it know it.

The term as used in the 2A obviously means "a militia that works well", kept so by the inability of the government to prevent the people from having the proper arms. i.e. if the government could disarm the people, you could not have a militia that works at all, and you need one of those for a free state.

I liken "well-regulated" to "well-tempered" in music, as in the "Well-Tempered Clavier" - a system of tuning.

Wikipedia has a rather good explanation on what "well tempered" and other forms of tuning mean.