But man does not live by the Silver Age alone. I was doing some rereading of mid 1970s Detective Comics and came across some things that I wanted to mention and put some markers down on. First, I want to establish the first time that Batman disappeared while Commissioner Gordon was in mid-sentence. This has become one of the cliches of the character and the relationship, but I am pretty sure that it never happened in the Golden or Silver Ages of Comics.
I'm not at all sure this is the first mention, but it's a good place to walk backwards from. Detective #449 (July 1975):
What about that giant tree in the Wayne Foundation HQ? I found this early image in Detective #463 (September 1976):
Check out this terrific three-panel sequence showing what happens when you try to mug the Batman dressed up as a woman:
Does Commissioner Gordon realize Bruce Wayne is Batman? Well, we see here that he might have some idea:
And given that we know from earlier issues that he correctly guessed his own daughter was Batgirl, it's no stretch to believe that Commissioner Gordon knew Bruce Wayne was really Batman. But this also hints at how increasingly complex the commish himself was becoming.