Yar,
R.P.O. 271 Radiator Overflow Condenser
(part# 3690732)
The installation instructions are found on Section 0 sheet 17.00 of the 1947-54 truck factory assembly manual.
It is called a condenser because, at the time, The concept was to catch the steam from the overflow (zero pressure or very low pressure cap)and cool it back to liquid in the metal can and let it be sucked in when the engine cooled. They didn't have the concept of a purged air cooling system at that time. But it works like any "modern" overflow tank when you keep it at least 1/2 full of coolant.
