Howdy
Traditionally, every S&W revolver made to shoot 45ACP had more space between the rear of the cylinder and the frame than any of their revolvers made to shoot 45 Colt.
This is because with a rimless cartridge case, the extractor would have nothing to grab to extract empty brass. So S&W developed half moon and full moon clips for the 45ACP ammo to be clipped to.
45 Colt rims are nominally .060 thick. The space between the rear of the cylinder and the frame is just a tad over .060 for a S&W revolver chambered for 45 Colt.
The space between the rear of the cylinder and the frame in a S&W revolver chambered for 45 ACP is about .030 more, to allow for the thickness of the clips.
Here is a photo of a couple of S&W Model 1917 revolvers, an original from 1918 at the top, and one from the Brazilian contract in the 1930s at the bottom. Notice how much space there is between the rear of the cylinder and the frame. A tad more than .090.
This Model 25-3 is chambered for 45 Colt. There is about .030 less space behind the cylinder. Only enough space for the 45 Colt rims, no clips needed.
At the top of this photo is the Model 25-3, at the bottom is a 1955 Target chambered for 45 ACP. If you look carefully you should be able to see there is more space behind the cylinder with the 45ACP revolver than the 45 Colt revolver.
In the 1930s the 45 Auto Rim cartridge was developed for the Model 1917 without the use of clips. The rim was about .090 thick, to make up for the absence of clips.This allowed the cartridges to be head spaced on the rims. The 45 Left to right in this photo the cartridges are 45 Colt, 45 Schofield, 45 Cowboy Special, 45 Auto Rim, and 45 ACP.
MCB is correct. A 45 Colt revolver could have the center of the rear of the cylinder relieved by .030 to allow 45ACP cartridges with clips to be seated. This is not possible with a 45 ACP revolver because 45 Colt rims would have .030 of slop when seated. As to why S&W has not offered this option, you would have to ask them.