I just picked up a Model 65-5 and noticed that the cylinder won't always lock once closed. It only happens if the cylinder is turned clockwise after it is closed and can often make a full rotation before it locks into position. Any idea what the problem could be?
Sounds like something is causing the "Hand" to stick. Carefully take off the side plate and use a solvent to clean the parts in place if you're not comfortable disassembling everything. Dry with a can of compressed air (the kind for computer keyboards) and then lightly lube the moving parts. Ensure the "hand" isn't binding. Reassemble and test.
If the stop is not holding it might just be accumulated crud or old oil around the cyl. stop and/or the spring. A good cleaning might solve the issue.
Also, using a magnifier if you have one, check the top of the ball of the cylinder stop for wear and damage, and the stop slots in the cylinder for accumulated dirt or peening damage.