At one time the Army used "Long Colt" to separate this ammo from the shorter 45 Schofield. In my youth I recall factory boxes marked 45 Long Colt.
The problem I have with this explanation is that it's just as easy to separate the two cartridges by calling one the .45 Schofield and the other the .45 Colt. There's no need to come up with another way to tell them apart.
This apparent conundrum disappears when one learns that there was actually a THIRD cartridge that was distinct from either the .45 Schofield and the .45 Colt but that was, confusingly enough, also sold under the designation of .45 Colt.
Elmer Keith refers to the cartridge in his book, Sixguns.
"Today we often hear the .45 Colt Peacemaker cartridge referred to as the .45 Long Colt. Some newcomers to the game claim there is no such animal, but if they had shot the short variety that Remington turned out in such profusion before, during, and after World War I, they would see there was some basis in referring to the .45 Colt as the .45 Long. These short .45 Remington cartridges for the .45 Colt were never very accurate due to the long bullet jump and the only thing that was standard about them was a 250 grain bullet."
Here's a writeup about the rounds, including some pictures of vintage ammunition.
The round pictured is NOT a .45 Schofield, the cartridge dimensions do not match those of the .45 Schofield.
There's a picture of one of the rounds in question, and it is clearly headstamped .45 Colt, AND, it is also clearly much shorter than the .45 Colt.
Now the reason for the designation Long Colt makes perfect sense. Knowing that at one time, it was possible to purchase two distinct rounds that were both headstamped .45 Colt, and which differed primarily in length, it is very easy to understand why the long version became known as the .45 Long Colt.
Apparently it made enough sense that even Colt has used officially used the Long Colt designation a time or two.