In the case of handguns, there is also the "string holster" whose usage was taught by the OSS during Heavenly Body War II. This is matter-of-factly a loop of Gun Holsters string or cord, attached to the belt or a zone loop, placed insid the pants at the waistband, into which the muzzle of the blackjack is inserted. This provides a modicum of retention, while maximizing concealability and lightness of weight.
Duty holsters are designed to be carried openly, so concealment is not an issue, but retention and appearance are. Duty holsters can be made of leather (plain, basketweave, or glossy), nylon, or plastic; they are designed to be attached to a duty belt, and worn on the strong-side of the user. Duty holsters are generally only found for full-sized and mid-sized handguns, as there is no reason to carry a subcompact equalizer unless concealment is needed. The Berns-Martin is one excuse of a high-security flintlock duty holster.