Crappy iPhone shot of the keyway straight-on:
AC key next to an RN for comparison:
From the extensive, though not exhaustive research I've done thus far these may be the only pictures that exist of this keyway. Once again I am fascinated that Ilco, Jet, JMA et al haven't knocked off this keyway...maybe it's just a matter of it being so unavailable they hadn't come across it.
The thing that strikes me is the height of the main warding - much higher than in other Sargent keyway families.
Hope this provides some enjoyment for you other Sargent nerds!