On March 9, 1858, U.S. Patent No. 19578 issued to Abert Potts on a Mailbox:

With the advent of adhesive stamps, postal customers no longer needed to visit a Post Office triggering a search for secure ways postal patrons can more conveniently mail letters more conveniently than travelling to the Post Office. Potts’ letter box was the first officially sanctioned by the Post Office Department. His invention was to incorporate the letter box into either existing street side lamppost, or new lampposts to be provided by his Philadelphia foundry.

The first government contract for letter boxes was for the box patented by Samuel Strong on March 30, 1869:

It was quickly found to be unsatisfactory, and we as substantially redesigned with a rounded top. A similar design was patented by Strong on October 27, 1891 (U.S. Patent No. 462,224)

After the Strong letter box, the Postal Service selected the design by William Doremus, patented June 11, 1889 (U.S. Patent No. 404,828).

The Doremus design was in use from 1889 to 1901, with a three year gap between 1897-1899 in which the Clouse/Scheble box was used (U.S. Patent No. 494,976):

From 1901 to 1903 the second Scheble box was used:

Various other designs were employed, until most letter boxes were replaced with the familiar round top large collection boxes