AVP,
Moon clips, half moon clips, etc., are flat pieces of spring steel that snap around the extractor groove on rimless .45 ACP ammo.
Half-moon clips were invented by Joseph Wesson when the US was getting ready to enter WW I. The gov't wanted revolvers that could chamber the .45 ACP round, and was looking for a way to get around the fact that the rimless round wouldn't extract. The half-moon clips solved that problem.
There's some indication that Wesson experimented with full-moon clips, but the half-moons were chosen because they would "nestle" better in the loading pouch, and clipped ammo could be packed at the ammo factory with much less wasted space in a box.
IIRC, ammo from the factory came packed in pasteboard cartons, with 24 rounds on clips.
Now, you CAN shoot .45 ACP in S&W revolvers (and most Colt Model 1917 revolvers, Colt also made revolvers for the Gov't during WW I), but you can't extract the cases without using a pencil or something similar to punch them out.
The last major loader of .45 Auto Rim ammo, Remington, declared the ammo obsolete a few years ago. Brass is, however, still available, you just have to load your own.
You can still find the occasional box of .45 AR at gunshows.