Sandy, gave you the right answer. Here is the link to the "latest" time I posted these drawings showing the difference from the original Square-Bridge (w/three 20-mm guns forward) and the revised Square-Bridge (w/two twin 40-mm mounts) ...
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viewtopic.php?f=49&t=8105&start=1000#p470105 ...
The three FLETCHERS with this configuration, AMMEN, MULLANY, and BUSH, were the first three SQUARE-BRIDGE FLETCHERS modified with forward twin 40-mm mounts. The work was done at Beth-SF (their builder) as part of their Post-Shakedown work. They didn't wait for the final plans from Gibbs-Cox/USN Boston Navy Yard and used this layout, which may have been based on an earlier scheme.
Why MULLANY never had her bridge modified like her surviving sister AMMEN, is a bit of a mystery. AMMEN was updated when she was recommissioned at Charleston Naval Shipyard in 1951. Her forward 40-mm bulwarks, along with some of the other bulwarks on the ship, were in poor shape and when they "repaired" those they fixed the bridge to the "standard". MULLANY didn't have that done when she was recommissioned, reason unknown. I'm pretty sure once they realized that the forward twin 40-mm mounts were being removed anyway and two hedgehogs replacing them, that there was no reason to modify the bridge. Both AMMEN and MULLANY were modified to the 4-Gun mod as well ... at different yards.