The weight is held in place by friction between the rod supporting it and the steel disk at upper left. To release the friction and lower the weight, you just squeeze against the disk with your thumb. To raise the weight, simply pull it up along the rod. There is a small spring between the weight and the disk to maintain friction when you release your thumb. The disk is loosely attached to the weight by a screw so it is free to move back and forth through a small angle.
The holes in the disk and weight are 1/64 inch larger than the rod. The principle of operation is that when a steel rod passes through a slightly oversize hole in another piece of steel, and rests at a small angle, friction will tightly jam it in place. I've used a similar device in an adjustable chair that supports at least 200 pounds.
I used a bandsaw and drill press to build this. If you don't have these tools available, you could build a simpler version using a short piece of steel or PVC pipe and a pair of end caps. For extra weight fill the pipe with fender washers, ball bearings, lead shot, pennies, or whatever. Any piece of steel about 1/8 to 1/4 inch thick with a slightly oversized hole should work well as a locking device. In this version I used a stainless steel rod, but an ordinary zinc-plated carbon steel rod also works.
The telescope isn't finished and I haven't used the counterweight yet while observing, but in testing it works like a charm!