Yes I'd say the little circular bowl was a moistening pad (sponge with added water) This was mighty useful in preventing horrid tasting (and probably very unhealthy) glue from stamps and envelopes getting on your tongue.
I've still got one, but don't need it any more as stamps and envelopes are self sticking.
Not that we even need many of those these days.

Regarding Lost World's comment above, the USMC tie bar was introduced as part of the 1957 revamping of USMC uniforms and insignia, which were still pretty much WW2-style until the change. Another change was the addition of crossed rifles to NCO chevrons. Both are still in service to this day.

Yes boys, you'll get free cigarettes in every box of C-Rations.

At first I was going to say Dress Blue C--blue trousers with blood stripe, long sleeve khaki shirt with tie, and the white barracks cover visible in the background--a uniform worn mostly by recruiters. But the lack of ribbons and shooting badges on the shirt has me thinking this is a khaki service A uniform minus the green or khaki jacket (khaki jackets were long gone by my time). And perhaps the barracks cover in the background is khaki and not white. I also find it odd he's not wearing a tie clasp, but perhaps those came later.

is what appears to be a bowl of sand. For sprinkling on documents still wet from the dip pen?

[About a hundred years too late for that, but perhaps a moistening pad for stamps and envelopes? -tterrace]