We call them moon clips because we are lazy.
The right name is full moon clips. This is because the original clips were called "Half Moon Clips" because of their shape. The 1917 revolvers were chambered for the .45ACP.
They do have the shoulder in the chamber to headspace on, and using two 3-rnd half moon clips meant you could extract all the empties with the normal extractor, so no other changes to a standard production gun were needed. (that was an important thing in terms of time and money, at that time)
They will still work without the clips, just that reloading is about as slow as a single action, since you have to poke out the empties by hand with a rod/stick/pencil, etc.
The 6rnd "Full Moon Clip" came along later. As did the 2rnd "quarter moon clips". Not certain just when, but I'm sure it was well after the original half moon clips.
Now, you can build a lever gun that will handle different lengths of cartridges, interchangeably. Its done all the time in .22 rimfire.
But you have to build it to do that, from the start. Converting an existing "long" cartridge action to shoot a shorter one isn't easy. Although, I suppose a model 94 in .30-30 and one in .44Mag shows that you can adapt the action or one, OR the other without a complete redesign.
The added complexity of the differences between the ACP and Colt case, and their methods of headspacing, I just don't see any levergun design that could run them interchangeably.
Having said that, some bright kid will come along with a design which actually will do it....but I ain't seen it yet, and I'd bet the cowboy gamers wouldn't let you play with it, if you did!