The info on the primary T7500 heatsink is all in this thread, but is spread w a y out. To summarize the Dell part numbers for the three versions of the primary heatsink for future readers:
U402F - the biggest and most desirable of the three; dissipates tons of heat. Four copper tubes are partially filled with liquid refrigerant (Freon) that boils to carry heat away from the CPU. The gaseous refrigerant condenses back to liquid in the finned section and flows back to the CPU. The four copper pipes protrude through the four holes in the black plastic Memory Shroud. As of March, 2016, out of stock with Dell Parts Dept. and unavailable on eBay.
U016F - about 2/3 the fins of U402F, but is still a boiling liquid to a gas refrigerant-based design. The four copper pipes don't reach the four holes in the black plastic Memory Shroud.
T021F - a design built from a block of solid aluminum with slots. About the same size or a bit smaller than U016D. The least desirable of the three; hottest running of the three designs.