Don't know about the sigs but the blanket statement re the availability of permanent ink felt tips is wrong. Permanent ink felt tips were invented in the 1950s and were widely marketed under the "Magic Marker" brand by the early 1960s. They were state of the art for the advertising industry by the late 1960s and sold in stationary stores. My mom was in advertising at the time and I recall going to her office as a little boy and being set up in the art department to play with the hundreds of markers. And trust me, I know from experience that they wrote on everything: walls, doors, floors...
