By "UK" are you referring to Underwater Kinetics? If so, I never heard of a model 600, and couldn't find one on their website. Where/how did you hear about this light?
I have a 20 year old UK1200 which is an 8D lantern with a pistol grip. I've determined that it uses the same interchangeable pistol grip / lantern grips that are used with the lights now being sold. The bulbs used are the same 13W and 18W T-2 1/4 S based bulbs as the new 8D series.