One must ask "why" would a "buscadero" rig have been developed in the pre1900 era?
Lets roll back a bit.
1860's: "scabbard" style holsters were the rage. Handguns were long barreled and generally cap & ball. These scabbard holsters fit on the standard thin leather military belts of the time.
1870's: Cartridges! And thus the cartridge belt. These are big 2.5" wide affairs. The meager scabbard holsters wont fit a big 2"+ wide belt (unless its hung on the billet at the front of a ranger style belt . . . Ive got a picture of such.) Folks from Mexico to Wyoming became smart and started the fad of the "fold over" holster that tucked into its own loops (i.e. Mexican loop holster, Texas jock strap, etc...). This allowed the holster to fit a wide variety of belt thickness.
These holsters styles lasted well into the 1890's. "Packing Iron" has many examples. This holster design was versatile (would fit many different belts of a variety of thicknesses), quick to deliver, and secure (especially on horseback). The gun remained in a vertical stance while mounted.
One must ask: what advantage or situation would've prompted a "drop leg" rig that would ride in a special proprietary belt that would cant the gun back while mounted to a near horizontal position, thus encouraging the revolver to fall out of the rig while at a gallop? I just dont see one.
We know the "fast-draw-high-noon" situation is the stuff of silver screen fantasy. Thats about the only place these "buscadero" rigs would serve man well.
BTW: Buscar = spanish verb "to search". Buscadero = noun "Man who searches" or "searcher". Apparently thats what one does when you reach down for your gun with one of these drop rigs on.