Thank you for the reply. I did a little more digging on the other ships built by Bethlehem-Fairfiled and launched about the same time as the "Robert J. Collier." I found two photos of the "William H. Jackson," launched ten days after the "Collier" and delivered four days after the "Collier."
Unfortunately, the ship, as seen in the photos, is not yet completed and is lacking armament. Even the gun tubs are absent from the flying bridge. However, the bow gun platform is clearly shown. I would have to believe, as you have suggested, the 20mm gun was mounted there instead of the 3" or 4" gun.
Many sources indicate that the armament on the Liberty Ships varied greatly by builder, type, and time period. Here we have a prime example of this! If it were not for the lieutenant's log, this detail may have never been known.