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Unless I'm wrong, a stock '73 wouldn't have had any coolant overflow container. Just an overflow hose routed towards the ground. Probably an aftermarket one that a previous owner fashioned in there? I did work a small aftermarket one into the nose of my '70 in a different place than that though.
BD

 

[Burst]

I purchased the smallest aftermarket coolant recovery kit bottle I could find, and oddly enough mounted it very close to where 72 GT's less expensive version is mounted! differance being in a pinch for cash his would be worth deposit return money, mine would be worthless.