I don't think there is anything like a standard patchbox release mechanism. I use the systems that match the type of rifle that I am making, and even releases that look similar on the outside may be radically different on the inside. There are almost as many mechanisms as there are gun makers. That is probably an exaggeration, but you get the idea. There are lots of different patchbox opening and release systems. I would use the one that is right for the rifle you are building. You have to examine the originals to get an idea how they did it. Even then, I will take a basic idea and implement it how it is easiest for me at the time, given the parts, materials, and tools that I have on hand.