US Navy Cover Series From 1930-S And 1940-S

Hi,
I have to wait to get my grandfather's collection shipped to me here in Okinawa, but I was able to briefly look at what he left for me while moving it out of old boxes that were falling apart into cleaner boxes. One of the big things I remember was that he had what was perhaps hundreds of First Day issues on envelopes with a US Navy theme from the 1930s and 1940s. Does anyone have any information on this issue so I could learn about this part of his collection while I wait to have it shipped? Thanks in advance.