See a couple of shots of the Trathen on Navsource
http://www.navsource.org/archives/05/0553002.jpg
http://www.navsource.org/archives/05/0553003.jpg
(Navsource doesn't allow hot-linking so I couldn't post the pictures directly)
The first is reported to be dated 1957.
I doubt if there were 20mm guns onboard in the mid-to-late 50s. Twenties had developed the reputation of being the weapon of last resort during WWII. When the 20s opened up, it was time to duck & cover. They made the sailors feel that they were doing something.
The 40s were also likely landed in favor of 3"/50 twins. These may have had a radar-controlled director on the mount. The photos do seem to show a pair of directors on the raised structure between the stacks. The 3-inch guns were in the raised tubs adjacent to the aft stack and in the raised tub aft - between the 53 and 54 mount.
If you look at the first photo you can see that the twin 40s that were below the bridge have been replaced by a pair of Hedgehog launchers and ready lockers. Look too at the main deck - waist. there are no 20mm guns, but the DC throwers are retained. The aft deck also is fairly clear except for a pair of roll-off racks.
Get a copy of the Anatomy of the Ship-The Sullivans. It has a lot of good information on a mid-to-late 50s Fletcher