In looking at that thread, I would say that most '73s had then OEM, but also some '72s too. Parts book might be the definitive answer, either OEM or an accessory kit. As '72 and '73 C/Y cars were pretty much the same, I'd suspect an OEM '73 reservoir would also fit a similar '72.
Only thing about NOS plastic is that it might become brittle and also be more fragile. Having something "fresher" might be best in the long run. DO note the orientation of the reservoir's level to the OEM items, though, as I've determined that is important to the correct functioning of the reservoir.
Initially, the coolant recovery caps had a softer rubber seal on the underside of the cap where it touches the radiator filler neck. Stant used some harder spring brass items, but some of them were prone to cracking (and therefore loosing the necessary "seal" for the system to work). As things have evolved, the current coolant recovery caps (now standard on everything) look pretty much like a normal cap from the earlier '70s, to me. On those earlier systems, the coolant jug was not pressurized, unlike many current production vehicles where the coolant level is checked in a pressurized jug.
As things have evolved, the only real benefit is not longer coolant life, but a place to add coolant without removing the radiator cap. Other than the need for a "catch can" for cars that go to the drag strip. On some cars, there will still be "air" in the upper radiator tank, by observation, regardless of what kind of radiator cap is on the system.
If the cooling system is clean and operating as designed, the need for a coolant recovery system is not really necessary, by observation. There CAN be some situations where one is good to have, though.
CBODY67