Those aren't all they're cracked-up to be.

Modern rope locks are made from formed & welded steel plates. Older ones were made with cast metal and are more prone to stress cracks, and ultimately failure. Another good reason to do an annual inspection! Never load you rope locks to more than the recommended 50 pounds - keep your sets balanced and 'in-weight'. The rope lock may be stronger, but do you really want to find-out? This is also why you don't want to use your rope locks for rope brakes -- a dynamic load like a moving set and/or counterweight carraige can place much more strain on a rope lock than you would ever imagine.