Gun belts with belt loops existed before 1900, but they were uncommon because at the end of the Civil War there were very few cartridge guns, so there were no cartridges to put into the belt loops.
Between 1873 and 1900 there were 191,999 Colt SAA's made. 115,499 1877 Lightenings, 41,999 1878 Frontiers, 31,000 1889 Navies, 130,999 new model Armies and Navies. Which is 511,496 cartridge guns from Colt alone between 1873 and 1900. Then you have S&W, Remington and a host of others.