Hi . It looks like a remote header tank , installed as the high point of the cooling system , allowing the system to " burp" and clear air locks .
My Federal P6B has no where to install an elevated header and little space for an overflow tank . I found a neat unpressurised plastic coolant bottle with a long dip pipe in my local scrap yard . This tucks behind the radiator , and is plumbed to the radiator filler vent connection . It collects any over flow from the cooling system as the engine heats up , and returns it via the vacuum relief built into the radiator filler cap, when the engine cools down .
After checking the radiator was full when cold I half filled the new overflow bottle and took off on a 150 mile proving run
. The level increased as the engine reached operating temperature and the radiator cap vented , remained constant at the new level with the engine running and went back to the original level as the engine cooled down. I now have a completely closed cooling system , which does not leak[ at least today !!] ; with an additional amount of reserve coolant available to make up for any small leaks which may occur in the future .
Hope this helps and shows there's no need to go looking for pressurised over flow tanks
Cheers
RVW