Herb,
I think you may be right that RGS signal boxes may not have included stencil displays. But I get the general impression that the two funcitons were sometimes combined as with the 4-wheel caboose shown in the link I posted. The descriptions of it says: "The miniature signal cupola boasts built-in marker lights and number boards." I see some of these boxes that have a round lens and a rectangular glass display board, as does the one referred to in the link.
I suspect the practice of displaying train numbers ended quite early, whereas markers survived up until the end of cabooses.