USS ATLANTA (and USS SAN DIEGO) had the SC-1 radar antenna of the foremast and no radar antenna on the mainmast. The standard authorized configuration was changed to the new SG radar antenna installed on the foremast and the SC-1 antenna installed on the mainmast for USS JUNEAU (and USS SAN JUAN) by the time of their completion.
USS ATLANTA had eight 20-mm guns by Midway and at least to late October 1942. Whether she had the boats removed and additional 20-mm guns installed in their place is unknown between late October and prior to her loss.
It isn't clear, but it appears that ATLANTA had three Mk 44 directors for her quad 1.1-in for the three mounts installed on the superstructure. It is less certain about the fantail quad 1.1-in since that mount was installed after shakedown.
USS ATLANTA had her entire hull repainted with 5-N (superstructure remained in Ms 12R(mod)) after Midway at PHNY.