Hull numbers were not displayed on battleships, cruisers, or aircraft carriers during the 1930s. Even after the war started, some cruisers when they were repainted or touched up, painted over the hull numbers for periods of time until they had time to reapply the hull numbers. Ship recognition was fairly easy to determine the ID of a given ship during those periods without the hull numbers (except for destroyers that had been carrying hull numbers since WWI). Once new ships were added to the Fleet with many units in each class, plus with wholesale "green crews" manning them, keeping hull numbers visible was a necessity.
Chuck,
If you are modeling USS SAN FRANCISCO (or any other NEW ORLEANS class cruiser for any time frame in 1939), the ship would have by and large looked as they did during the late 1930's. The upgrades associated with WWII (the King Board Air Defense Improvements and things like installing radar) didn't start to appear on ships until mid-1940. For some ships they didn't START to get these mods until 1941 (very true for destroyers). The King Board ADI mods not only involved adding AA armament and splinter shielding, but also included structural mods to improve firing arcs and as weight compensation removals. Radars were slow to appear with the CXAM radars being the first "serial" installations, and they were on very few ships, some carriers, battleships and cruisers. Application of camouflage painting started in the Spring of 1941.
After WWII started in Europe in September 1939, the USN started to be restrictive on photos being taken of their ships as mods took place. There wasn't a "legal" ban on taking photos by civilians, media, etc, but they tried to "encourage" them to NOT take and release photos. This is why there are fewer images of USN ships in the 1940-41 period available. The World's Fair in 1940 was one of the last times when there were quite a few photos of USN ships taken and published by non-USN sources. By and large the photos taken then show no differences in configuration from the previous few years. Some media, LIFE magazine is an example, under USN "supervision" were allowed to take images of USN ships for PR purposes.