It's a board with a little tube. When the light is on the tube lights up and lets you see the lens aperture. At least in theory. My beseler came supposedly from a well known Canadian art photographer. One of the mods he made to the thing was to paint all the pilot lights so they didn't transmit any light. I think the newer Rodenstock and Schendier lenses actually light up. I've got mostly Nikons and an older Rodenstock. These don't light up and I've never felt the need to scrape the paint off the lensboards.
Get your self a threaded 39mm board. Then you can just unscrew the lens when you need to change things. Changing a Beseler board isn't hard but unscrewing a lens is even easier.