The K boxes show it as K-xx Masterpiece . . . Go figure why some sources call it a Target Masterpiece. S&W did not. Probably for the same reason many people insist on calling the .45 Colt cartridge a .45 Long Colt when there actually is no such thing. Someone who knew no better just started calling it that, it wasn't corrected, and the name caught on.
I usually rely on company printed materials for who "was" right. I say was because you have to look at the time period the gun in question was made. Over time, the company has changed the names of their products. The 38 HE line is a good example of changing model names at least 4 times before the introduction of model numbers.
The only 2 reasons for the emergence of the term 45 Long Colt that I lend any credibility to are when comparing to 45 Schofield or complementing the 38 Long Colt.
In the 1800s, the Frankford Arsenal was basically labeling the two 45 cartridges the same, Colt Revolver, Cal; .45, or Revolver Ball Cartridges, Calibre, .45. How did the military or private individuals order ammo? Did you take your chances, or order the long cartridge for Colt's and the short cartridge for S&W?
The more logical association to the 45 Long Colt term would be to compliment the 38 Long Colt, plus easily differentiate that cartridge from the shorter 45 Schofield, various short British 45s, or the 45 ACP round in the early 1900s.
It even caught on with some ammo manufacturers.