Embarrassing question

Sorry, I’m happy to be laughed at here, but I can’t find the info anywhere. 

Just bought a SC living saloon and was checking fluids and noticed two radiator caps. I just can’t figure out what the top one is doing. 
And when I opened it up it was empty. Don’t really want to pour anything in there without knowing what it actually is. 

https://i.servimg.com/u/f32/20/51/60/35/img_0111.jpg

Thanks a a lot!

Looks like a factory inter-cooler: https://forum.ih8mud.com/threads/factory-air-water-intercooled-1kzte.944624/

The little yellow cap with the two medium sided double pipes is the top of the cooling system, one pipe goes to an expansion bottle, that in general will need to be cleaned out and refilled.

Then as the engine warms up the level in the expansion bottle rises, when it coold the level falls, so there's a mark of 'fill it here when cold' on it somewhere - perhaps a min and max - only fill to the max mark when the engine is cold.

The big black cap is the power steering, to the RHS of that is the fuel primer pump (only used if you drain the tank).