I remember seeing those devices also, either with a plastic body mounted in a metal plate, or else made entirely of porcelain, that would fit in place of the blank cover. I've seen them for edison base lamp sockets, light switches (I think the "despard" type), single and double U-ground and non-grounded receptacles, and even some that simply had three or four small holes for individual wires to pass through (for transitioning from Knob and Tube to Conduit...who knows?). I'm not sure that any of these devices were intended for outdoor use.
These things were available many years ago, and I think they were made by Appleton. I wonder if these devices would even be permitted today under the 2002 NEC.
Someone please update me on this. Thanks.