[rocketraider]

Overflow jugs showed up in 1970 as part of HD cooling system option, which was required with factory trailer package options and I think (not 100% sure) W30. Emissions calibrated engines ran leaner and hotter was biggest reason they appeared, though they'd been used on over-the-road trucks for years before then.

You can still find aftermarket overflow systems (see Allan's pic). They work, but finding a place to put them can be a problem. I remember putting one on my dad's 68 El Camino (that always had a chronic puking problem no matter where you set the radiator level) and we put the jug in the washer jar bracket since, like most GM washers, it had quit working about a year after he bought it new.