Questions on Atlanta class ships:
Several photos including navsource.org show different bridges on the Atlantas. Some round, some stepped round, even a modified open round? and then the square bridges of the Oakland class'. What was the rhyme or reason to these different bridges? Upgrades during service life or different shipyard specs during construction?
Several 'Atlanta's have early boat decks similar to pre-war ships and some don't. Again, an upgrade or construction difference?
There were 3 batches of Atlanta's, batch 1 - CL 51-54, batch 2 - CL 95-98, and batch 3 - CL 119-121. The first 2 batches were very similar. Batch 1 (Atlanta's) ALL had the stepped bridge. Some confusion arose from the B&W photos (see NH-97819 of San Juan on navsource) because the step was not always visible from some angles, even though it was always there. The open bridge on top of the pilothouse (PH) changed from a rudimentary thing that did not extend to the front of the PH, to the more extensive thing that followed the curve of the PH (San Diego and San Juan only). San Diego had splinter screens added at the step in front of the PH, as part of a "screen flagship" mod.
Batch 2 (Oakland's) actually had the same basic bridge. However, this was disguised by the fact that the PH sides were "shaved off" to narrow the structure, and simplify construction. (Flat plate is faster - fewer steps in the construction process). The PH face was still curved, as in the first 4. The curved front to the lower step was disguised by the forward extension on which 2 20MM mounts were placed. But the underlying structure remained very similar to the original batch. The open bridge, on top of the PH, was flat plate, again for simplicity of construction.
Batch 3 had the superstructure totally redesigned to lower the CG, and allow more AA guns to be mounted. On this group, the design team really did start from scratch, and incorporated all of the lessons of the war to date. 2/3 of the main battery were lowered by 1 deck level, and the bridge and fore-funnel moved aft to make room for a quad 40MM. This batch had even fewer curved plates in the superstructure.
The first 2 batches were designed for boats amidships. Only the first group carried them, though, and they were gradually removed to create space and weight compensation for added AA. Eventually, the crane went, as well. The second batch incorporated these changes from their completion. San Juan and all of the second batch carried 2 40MM twins in the former boat stowage area.