Latitude Zero and The Green Slime are also in the running for most Westerners; I think they're Toho, but may have been coproductions intended for the international market in the first place.
No need for shame. I decided that after carefully curating my G's for quality I needed them all, and scooped up the 70s entries when the getting was good. At the very least I would expect all of these to be reissued next year. Note that a R3 DVD of THE RETURN OF GODZILLA, minus Burr, and those...
Come back in a year, when the next US GODZILLA should spur interest in its predecessors. (Those DVDs are nice.) There are apparently some beautiful Blus of classic kaiju available from Japan but they're expensive and unsubtitled.