Looking at my dash vents more closely for the first time ... looks like a PO put in the later dash vents from a '75 or '76 (the ones with the little lever at the bottom instead of the dial).
Thus, I'm looking for a pair of excellent condition dash vents for a '73. No cracks, no missing spring tabs, working directional mechanicals, etc.
Happy to trade someone for the later dash vents I have, which are both in excellent condition.