sandpaper finish? I have two questions:
1 - could you please take a photo of the sign with a flash, to see if it lights up?
2 - could you estimate the grain of the sandpaper-like texture? like, 40 grain, or 200?
the reason I ask these questions is because I suspect the finish is actually tiny glass beads - an early attempt at reflectivity. Before the sheeting we know today was invented, signs were roll-coated with clear paint with glass beads immersed in it. As the years went by, the beads got smaller and smaller, as they would adhere better and the reflectivity would be more durable.
here is an example of a route marker with the beads rolled on:
my guess is, that if the STOP sign was mounted to a 1930s bus, it has some fairly large beads. Or, the bus sign may have originally been non-reflective, and then retrofitted with the beads later.