Wonderful video on the darkroom of Adams, thanks !
Leitz on this lamp head, in their catalogue about the Vasex enlargers: "If, however, complete diffused light is desired for special purposes, as for instance to soften the appearance of much retouched plates, a special diffused light system can be fitted for the illumination system. In this alteration the upper hemisphere with its 75-watt opal lamp is simply exchanged for another with four ordinary 60-watt lamps, and a special combination of matt-surfaced an opal discs put in place of the two removable condenser lenses"
In other words, it was an accessory lamp head & diffuser meant to solve high contrast issues. The Vasex enlargers went up to 6X9 and I doubt if switching off one bulb makes any change in only one part of the image. When I make tests, I will have a look if even illumination is affected by turning off one or two lamps.