Somewhere before 1916, it became apparent to S&W that two things were about to happen.
First, the USA was going to get involved in WW I. Second, there would be a shortage of the 1911 pistol. Joseph Wesson set his engineers to build a revolver that would handle the 45 ACP round. The Second Model Hand Ejector was already being produced for Great Britain so they started there. Getting a cylinder to chamber and fire the ACP cartridge was easy, just chamber the cylinder properly to headspace on the case mouth. The hard part was extracting the fired cartridges. Tilting the muzzle up often allowed the fired cartridges to just fall out with a little shaking. Or a flick of a nail or knife blade would get them loose also. It was thought, correctly, that the Army would not look on those as acceptable methods. Joe Wesson put a couple of folks to work on it, one being himself. Two things were found and patented, a 3 round clip, the half moon with which we are familiar, and a 6 round, full moon clip. This full moon clip was not the one that S&W came up with 60 years later but a less robust version with spring fingers to hold the cartridges. (They were made, tested and failed by the Army.)
The 1/2 moon clip was made for one purpose only, to allow the spent cartridges to be extracted from the cylinder chambers. The were not built to headspace the cartridge. Headspacing was controlled by the depth of the chamber.
It worked and S&W allowed Colts to use the 1/2 moon clips in the revolvers they produced. Actually, in the early Colt revolvers, the chambers were bored straight through, so in the Colt revolvers, headspace was controlled by the clips. The Army soon caused Colt to correct that error and fix the revolvers that had already been accepted.
Fast forward to the 1970s. S&W approaches Ranch Products in Malinta, Ohio. They want Ranch Products to produce a new 6 shot, full moon clip and provide the drawings to be used to build them. These were to be used in the Model 25-2 since no other ACP revolver was produced by S&W during that era.
The Model 1988 was a stainless ACP revolver and sometime after that, S&W got lazy and stopped boring the cylinders with the attention to detail required to headspace ACP ammunition in a revolver. In those revolvers, moon clips are needed.
Kevin