World War Two censorship related labels, markings, and enclosures like the two in this thread are documented in
Civil Censorship in the United States During World War II", by Wilfrid N. Broderick and Dann Mayo, published by the Civil Censorship Study Group (CCSG) and War Cover Club in 1980. Although out of print, I believe a photocopy or PDF may still be available from the CCSG.
This reference is essential for anyone interested in
United States civilian censorship during WW-II. [Edited to include United States]
The two labels illustrated here are described on pages 92 and 93 as Types F15.1 and F15.2:
Page 92...

Page 93...

Don's OC-15 form was probably applied at the New York censorship office.
Linus's form was probably applied at the Seattle censorship office.
Mike