Typically there are some variations from a lead class ship and her following sisters.
From what research I did, these are some differences between Iowa and her followers:
 
The Iowa had three levels in her conning tower versus two levels in her sisters (BB62 through BB66). 
-The lower level was for a flag control platform, the middle level as a ship control station and the upper level being a fire control station for the Mark 40 (main battery) director. 
-The lower, or Flag Level of the conning tower, is omitted in BB62 through BB66.
- The Iowa was fitted with 20-mm mounts atop #2 turret so as not to block the view from the flag control level of her conning tower.
Subtle differnces at best, perhaps, and when more information of the actualy kit becomes available, MSW will report on them.