As Askia notes, it’s in the supplemental material, specifically a letter to Sally from her agent and future husband, Larry Schexnayder. The letter, which is in the supplement to #9 (just before the Playboy interview), is dated February 3rd, 1948.
Nelly is Lieutenant Nelson Gardner, USMC, a/k/a Captain Metropolis. His identity is revealed on the previous page. H.J. is Hooded Justice, obviously.