What is it?
 

The tank where the thermostat housing would be. Where the old schoolers at???

http://i261.photobucket.com/albums/i...a/engine-1.jpg

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That car is missing a stock C3 expansion tank hookup, from what I can tell. I think that's a homemade expansion setup.

What year vehicle? If it is '68-'72, then it's definetly missing something. Looks like 73+ though.

I's a filler neck expansion tank. http://www.jegs.com/i/JEGS-Performan...rentProductId=

Thank you, gentlemen. It was in an inquiry thread on ls1lt1 8-)

Used when the radiator is below the thermostat housing.. Prevents air bubbles..

Not all 68-72's used an expansion tank.
Certain engine options didn't use them.

You're correct on the 73 and up model year. Judging by the hood catches it isn't later then a 1976 model year.