I bought one of these when I started diving (which was sometime in 1984). So this light, which has seen little use is now somewhat of a fossil, but a powerful albeit heavy light. It uses 4 D cell batteries.
My immediate problem is replacing the bulb. It is the same bulb as used in the newer D4 model lights. In 2012 I see them for sale for $11.00 at "divers supply" online and just ordered two. It is a Xenon filled 5.8 Watt 6 volt bipin bulb. List price for this bulb is $23.95 It is UK Part number 44806.
I am tempted to buy some G4 base (the 4mm spacing certainly matches the bulb in my UK400) 6 volt 5 watt bulbs "T2.75" from 1000bulbs.com. These cost $1.76 each and crank out 60 lumens with a 2000 hour life. So I placed the order - these are Hikari JC-5603 bulbs. I'll let you know if and how they work in the light when they arrive.
I have run across a number of descriptions online of people modifying this light to use some kind of LED module, and a lighter set of rechargeable NiMH batteries, and I may look into this once my bulbs all burn out.