Green’s Dictionary of Slang

(US gay/prison) any form of note, letter, etc.

1935		A.J. Pollock Und. Speaks 15/1: Bull, a dollar publication (prison).
1972		(ref. to 1940s) B. Rodgers Queens’ Vernacular 38: bull (’40s) [...] 2. (prison sl, fr bulletin) note, letter.