This 100 watt Tungsram modeling lamp is a replacement for the lamp that is provided with the 2100C,2105C,2110C,2120C,and 2140C flash heads. This uniquely shaped lamp(that some refer to as a "mushroom") does not extend too far beyond the rim of the reflector. It is large enough to provide plenty of light, while being small enough to not block any of the output from its neighboring flash tube.
Since it is so inexpensive,some professional photographers buy a box of them, so they always have extras on hand. It allows them to keep their modeling lamps on,in the studio,throughout the workday.
Since it does not creat a great deal of heat,the flash heads are able to be cooled through natural convection.