"Closed" versus "Open" is a long conversation, best over a couple beers with a set of heads on the bench. Aside from the physical difference, there is a difference in flame propagation and combustion efficiency that makes open chamber heads superior to closed chamber.
As for identifyiong heads, the date code will tell you for which year a set of heads was used. Production for any particular year was started during July and August (usually) the previous year.
The only heads that will cause you heartache are the #15 and #16 from 1968 and 1970. Both the #15 heads had press-in studs. The #16 heads from 1970 had press-in studs. The #16 heads from 1968 had screw-in studs, and they are the good ones.
Another possible problem comes from the #46 heads, which were around in 1969 (On 428's) and in 1973 (on 350's).
