It's one of those "disambiguation's" , that has morphed itself to become acceptable mainstream slang .
It allowed for the uniformed to differentiate between 45 Colt & 45 ACP and less degree 45 Schofield.
References to .45 Long Colt appeared before the introduction of the .45 ACP. The term was sporadically used by quartermasters in the late 1870s and early 1880s to differentiate between full length Colt .45 cartridges and the dual use .45 Colt/Schofield cartridge until stocks of the full length cartridge were used up.
The Franklin Arsenal initially produced full length .45 Colt cartridges loaded with a 250gr bullet and 30gr of BP. It stopped manufacturing them in August 1874. Beginning in early 1875 it began issuing the "Revolver Ball Cartridge, Caliber .45". This was a compromise cartridge that would chamber in both the Colt M1873 and the S&W #3 (Schofield). It has an OL 0.19" shorter than the Colt, with a smaller rim than the .45 Schofield cartridge, and was loaded with a 203gr RNFP bullet and 28gr BP.
The "Revolver Ball Cartridge, Caliber .45" was originally a Berdan primed cartridge. In 1882 it was converted to Boxer priming and was superseded in 1887 by the M1887 ball cartridge (also a dual use cartridge).
Of note, the Colt .45 cartridge used in the M1873 field trials used a load of 40gr BP behind a 255 gr bullet.
Colt's Revolver .45 cartridge:
Revolver Ball Cartridge, Caliber .45
Ref:
Hackley, Woodin and Scranton, “History of Modern U. S. Military Small Arms Ammunition” Volume 1
McChristian, "THe U.S. Army in the West, 1870-1880 Uniforms, Weapons, and Equipment"
http://www.oldammo.com/november14.htm