I have never seen a Randall shop-supplied rig like this one. The very first Pathfinders (before they were a catalog Model #) came in a variety of sheaths. Sullivans, like the one you have were used fairly early but some of the old Johnson (Johnny Johnson) pouch sheaths (sheath type / no keeper strap) were used early on. I hate to speculate as to why, how, who, etc...but they could well have been spare sheaths, laying around the Randall shop...not being model-specific, and working quite well.
I'll dig up an example and post it later on. Right now, and for the past 2 days, I have been working on a new "redneck photo-shootin' project"...more to come...
Best, Capt. Chris