Nice registered cover.
However, you should note that the Third Assistant Postmaster General had been a long established office of the US Post Office Department, and when stamps came into existence, they fell under his jurisdiction:
Quote:
The Third Assistant Postmaster General's Office was created by Congress on 2 July 1836 to "provide more effectually for the settlement of the accounts" of the Post Office Department (POD). All of the Department's financial and accounting operations came under its charge, including collecting revenues from postmasters; paying the Department's contractors; and administering the dead letter office, registered mail service (introduced 1855), postal money orders (1864), and the postal savings system (1911).
Likewise, when postage stamps were introduced in 1847 they fell under the Third Assistant's jurisdiction as accountable paper. The quantity and variety of postal paper produced by the POD steadily increased—including stamped envelopes (introduced 1853), letter sheets and periodical wrappers (1861), and postal cards (1873)—and in 1874 a separate Division of Stamps was organized within the Third Assistant's Office to manage their production and distribution.