4"x 4" lens board with 5/8" extension, 42mm hole and "pilot light". These extended lensboards are typically used for mounting lenses with 42mm mounting thread, such as certain Schneider Componon-S 135 and 150mm lenses. The lensboards are not threaded - a 42mm retaining ring (jam nut) is required to mount the lens. These lens boards are used and may have cosmetic marks from previous use, but are certified to be in perfect working condition.
Note: The "pilot light" is intended to cast a small amount of red light on the aperture scale of the enlarging lens. This may be helpful when using lenses that do not themselves have an illuminated aperture scale feature. However, if the light is not required or wanted, it's easy to block the light by placing a small piece of black tape over the light pipe on the rear of the lensboard.