But surely *both* the Earth gates pre-date the Goa'uld invasion; the refugee Atlanteans would hardly have built a new one just for Ra to use, would they? Not to mention that any Gates made in that period would probably have included Atlantis-level tech.

Unless Ra brought that one with him from some other planet. In which case the triangle-and-circle PoO wouldn't have originally been Earth's at all. I'm going to start confusing myself again.