OK ... I'm back from NARA, I was there when these questions came up, and after a bunch of other things to attend to, now I can look at my references in detail.
I don't know where to start, but I will start with when USS PERKINS (DD-377) received her twin 40-mm mounts. Both mounts were installed during October 1942 at PHNY. Her yard period began on 20 October and was scheduled to complete on 3 November 1942. She was scheduled to receive besides two twin 40-mm mounts, a FD (Mk4) radar, SG radar, and BL-1 IFF.
I have scanned the 25 February 1942 image of PERKINS taken by USS TANGIER (AV-8). By that time she had SC radar installed but not FD (Mk 4) radar installed and the midships 53 mount removed. Blowing up the TANGIER image PERKINS "looks" to have SIX 20-mm guns at that time. Two 20-mm guns forward of the bridge, two abreast the second funnel (with only railings and no bulwark or canvas covers), and two on the aft deckhouse. I still think the gun in the elevated position on the forward end of the aft deckhouse was a 50-cal MG. By July 1942, sister USS CUSHING (DD-376) after her MINY period, had SEVEN 20-mm guns, the suspected 50-cal MG was replaced with a seventh 20-mm gun. I suspect that PERKINS also had by then replaced the 50-cal MG with a 20-mm gun.
Camo for PERKINS; the February 1942 photo appears to show Ms 21 (technically it was called Ms 11 with 5-N instead of 5-S). It is "possible" that USS PERKINS was painted in Ms 11 with 5-S in February 1942, but unlikely given that she was at MINY starting on 15 January 1942 for installation of radar (removal of the 53 mount and addition of the 20-mm guns) and returned to Pearl Harbor by 25 January. The USN had firmly decided that Ms 11 with 5-N was to be applied in mid-December 1941 and that is what MINY would have applied.
As a contrast to the configuration of USS PERKINS and USS CUSHING (along with USS SMITH (DD-378) and USS PRESTON (DD-379) they formed DesDiv 10), this September 1942 photo of USS REID (DD-369) shows a different arrangement to the aft deckhouse and a total of EIGHT 20-mm guns; two forward of the bridge, two abreast the second funnel, two on the forward end of the aft deckhouse and two atop the small deckhouse on the aft deckhouse. Neither PERKINS or CUSHING would have been modified to this arrangement prior to getting twin 40-mm guns once they had such structures removed. Note that in NONE of these photos, has the FD (Mk 4) radar been installed yet.