this sounds like a colour enlarger (although to my knowledge JOBO called the opal b/w version 6600 and the colour version 6700). The infos I have from Jobos site says that an external transformer is needed, in this case a 12V 100W model. A transformer, either classic or electronic for halogen lighting systems should do fine. Wattage can be higher but not lower.
Please make sure that there is no transformer in the head. The wiring should be visible when you open it.
The marking on the plug (when molded in the plastic) could be simply the max. rating of the plug itself and not the operating voltage.
P.S. goto www.jobo.com, contact Jobo and ask for a manual.