Or to give the original title for the Austrian patent granted in 1911; “Sicherheitsvorrichtung für Rasiermesser”. A more direct translation would be “Safety contraption for shaving knife”.
The original approach – as exemplified by the 1762 Perrett’s safety razor – was to place a guard on a straight razor. The idea were only slightly changed in the guise of Paul Zammet’s Improved Razor Guard. And in 1911 Thomas Claude Durham made another incremental improvement. Well, that and a bit more.
The intention with the patent was to come up with a safety guard that could not accidentally be pulled of the razor. This was done, according to the patent, by having a safety device that required a large amount of force to dismount.
The device took the form of a guard bar, that would act in much the same way as the guard on a regular safety razor.
Equally interesting is the device shown in figure 2 on the drawing; the device mounted on a shavette that Durham also patented. It used a slotted blade held in place with a sliding, dovetailed cap. Overall the shavette seems well though out and reasonable easy to manufacture. As shown in the patent, it had it’s own built in comb guard.
Both patents by Thomas Claude Durham can be read on Espace.net with machine translated copies on Google Patents.