Thank you Shawn. Google patents is truly a wonderful tool for researching holster innovations. The "citations" and "cited by" links provide abundant rabbit holes for inquiring minds. Yet, the "patent record" is much like the geologic "fossil record". It is discontinuous and incomplete.

Perhaps, an example is the patent recently granted to Spencer Keeper relating to his kydex appendix holster.

US9222750B1 - Holstering a gun
- Google Patents

Certainly, pistols have been carried forward of the hip inside the belt since the development of the flintlock. Interestingly, Keeper does not cite any previously patented "appendix holsters". Nichols' Bianchi 3S adjustable snap-loop IWB is cited and the design accommodates appendix carry, though not its intended purpose. In fairness to Keeper, the primary basis of his patent claim appears to be the construction of the mouth of the holster, which is intended to reduce the potential for negligent discharges.

So, who developed the first "modern" holster for the purpose of carrying a pistol forward of the hip? Many would point to Bruce Nelson's original Summer Special. Nelson didn't patent his designs, however.

Likewise, it seems the "origin" of the ankle holster for carrying firearms may be hidden in the obscurity that the patent office refers to as "prior art."