On my 121 ('66) I adapt a overflow bottle, but with a 0 psi cap (only a cover)on the bottle as a safety due the rad come from a '62 car. So far so good (no snow on my city on winter,and like california on summer).

(image on http://www.brickboard.com/GALLERY/images/3954.jpg)

Joaquin
121 Amazon - Perú

UB;

George has it right...because its the pressure cap which determines the pressure...and both early and later systems certainly have one of those!....the question you should be asking is not "Closed vs. open", but "open overflow or overflow bottle"...and since overflow bottles are a dime a dozen, and they prevent you from loosing coolant, and the upgrade is simple and cheap, I recommend it.

See: http://sw-em.com/service%20notes.htm#Cooling%20System

George; I kinda dissagree with your assertion that the overflow bottle is a "reservoir of potentially cooler water"...because the system expands INTO the bottle as it heats and and the only time any (of that cooler water) comes back into the cooling system is as it cools down after shutdown...so there's not much to be gained in the way of cooling capacity...however, the overflow bottle does selfbleed all the air out of the rad, so the IS a slight gain in cooling system effectiveness there...

Cheers