Bob Schieffer in his book This Just In: What I Couldn’t Tell You on TV
tells how the Pentagon in the early 70s was open to the public (at least the outer rings). There were no security checks or clearances needed. The public could wander in and out at their pleasure. Schieffer even says he didn’t even bother to obtain his press credentials for the first few months of service as the Pentagon correspondent for CBS.
This Just In: What I Couldn’t Tell You on TV