Wartime Covers

My first thought was syrup but then I considered that might leave you with a sticky situation.

Like this?

Wow! Thank you all for the info and examples!! I know now what to look for.

1944 USS Oahu ARG-5 (Repair Ship) air mail censored cover:

Another type of censored US mail from WW 2 is "V-Mail". V-Mail was a faster way for a soldier or sailor to send a letter back home or to send a letter to your soldier or sailor overseas, and it also served the war effort by reducing the space and weight of mail on the limited number of transport aircraft flying across the oceans during the war. The way it worked was you used a widely available V-Mail form on which to write your letter. It would be censored and then microfilmed on a roll of film by the Army or Navy postal units. The postal unit gave first preference to the V-Mail forms, sorted them by destination, then microfilmed them on a roll of film. These rolls of microfilm were then put on the next cargo plane going to the general area of where it was being sent where the local V-mail postal unit printed them on paper (usually as about 80% of the size of the original letter) and put them into V-Mail marked envelopes that were put into the mail to go to their destination. V-Mail was given priority over regular cards and letters since they could put so many letters home on very small and lightweight film. Some numbers are that 150,000 regular cards and letters would weigh about 1,500 pounds and fill 22 extra large mail sacks could be put on microfilm weighing 45 pounds and filling one small mail sack. Back then cargo space was very limited and every pound counted. There were officers who were tasked with agonizing over every pound and cubic foot of cargo space on the limited number of aircraft that were flying across the oceans to the theaters of operation and making decisions on what could go by air what had to go on cargo ships so this savings was huge.

There are many varieties of V-mail forms and letters but here is an image of what two versions of V-mail covers look like followed by what an enclosed re-printed letter looks like. The last image is of a war time poster that was advertising V-mail to the public to get them to use this service: