And his Automag in an X15
And his M29 shoulder holster was by Bucheimer's sister company in L.A., Bucheimer-Clark, that was formed by a joining with Earl Clark's company inherited from his father E.E. Clark, prominent holster inventor. Different designers, different founding dates, different product lines, different factories, etc., etc. See a better explanation in the book Holstory.
Underarm shoulder holsters are a really really bad choice for big guns including the M29. For hunting, use the bandolier style instead - Bianchi makes the only good one - or a crossdraw belt holster, of which Bianchi makes the Cyclone and Galco makes a copy.
Shoulder holsters are for small pistols. Compact size, light in weight. Film actors only wear their concealment sets when the scene exposes it. Its why propmasters and costumers ike easy on and off paddle holsters and shoulder holsters. Not real life.