I think yer right Dakota. About the earliest example I found in Packing Iron in regards to a Mexican Loop is circa 1875 (page 96), and St. George is right about them being used much earlier than 1900, and to mention that book. It is probably the best reference work regarding gunleather ever published. He is also correct about cartridge belts spurring the advance of gunleather technology into the Mexican Loop. I myself decided to go with "Slim Jims" mainly due to the period I'm trying to emulate, the early 1870's (about 1872), when such holsters were more prevalent (though according to Packing Iron, they were used well into the 90's). Heck, I probably won't even use a belt with loops, period. The "correct" holster for you depends on what time-frame you're placing your persona into -- if that's a consideration for you, that is.
Happy Trails!