A quick posting on the evolution of the 20-mm gun locations on the main deck amidships on the Fletcher class destroyers.
This sketch isn't an exhaustive study of all the "exact" locations on the main deck amidships that 20-mm guns were mounted. I have mentioned how the number mounted and locations for these guns were moved around as production progressed and various builders added their own "styles".
A: The location of the two per side 20-mm guns that the first Bath Iron Works and Federal SB ships (DD445-451 at least) were delivered with. A weird side note, the early built Bethlehem SB - SF destroyers (I think all of DD526-528), among the first square bridge Fletchers, used this same location. Which by that time was NOT the authorized location. All but the Ammen (DD527) had the locations moved forward when they were updated. The Ammen however retained this location, it appears until the end of the war.
B: The location of the single 20-mm gun per side when the number of 20-mm guns was reduced to four from six (retaining the two on 01 deck before the bridge) when the fantail twin 40-mm mount was installed. Note that the aft most gun was removed.
C: In response to the demand for more 20-mm guns, the amidships main deck 20-mm guns were again brought back to two per side, but instead of mounting the 20-mm back where it had been located, it was installed ahead of the one that remained. From the BIW drawings the aft most 20-mm gun was mounted a little more forward that before on NEW construction ships. I suspect that the ships that had their 20-mm guns reduced in number, and then had it added back, didn't move the remaining gun and simply added the additional gun before it.
D and E: These two drawings show the two most common ways that 20-mm guns were installed once the admidships twin 40-mm mounts were installed. D shows the style/method used by many builders and simply continued the method already being used. Some yards, not necessarily builders, built or modified many ships to the method seen in E, where the bulwark was not curved inwards, but was butted up to the clipping rooms and blended in to it.
Don't get too excited about the EXACT locations where the 20-mm guns were mounted to the deck. It seems that there were some variations on where they were installed. These simple drawings are based on the Bath Iron Works Engineering Drawings and other yards could be slightly different.
Also, I included the original arbor post storage method and K-gun locations used on the DD449-451 (at least). Further down is shown a K-gun with the roller rack storsge method. I didn't try to show the variations in how these were done.
Also, I have not included the six ships that either were equipped with the cataplut or were intended to be, because they had at least to different layouts for their 20-mm guns. Also, I didn't show the twin 20-mm installation seen in 1945.
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Ship camoflage - saved. 
