IIRC, Charles Heard credited Rodd Redwing for, if not first, at about the same time, using metal in his holsters.

...Found it .... Handgun Leather, Guns & Ammo, March, 1959

"Redwing recalled that one of the old time badmen had used ladies' corset stays in his holster's mouth to keep it from binding the gun, so Rodd began making holsters for himself with flat spring-steel in his holster's lip. This served his purpose of holding the "mouth" open when he drew and as he was spinning his pistol back into the holster after one of his fine exhibitions of gun-twirling. But it remained for Arvo Ojala, who came to Hollywood in 1951,
to make the most sweeping/ changes of all in re-designing the Western rig."

(Actual year of development not given.)