Here is a link to very early cabooses.
[www.nyow.org]

Down the page, there is a four-wheel bobber with what appears to be a stencil box. It is referred to as a “signal cupola.” Elsewhere, there is a reference to “number boards.” A stencil box would be very similar to locomotive number boards with their illumination for nighttime conspicuity. And like stencil boxes, locomotive number boards would have changeable number stencils. However, my understanding is that caboose stencil boxes were not meant to display the road number of the caboose as a locomotive number board displays the road number of the locomotive. Caboose stencil boxes displayed train numbers.