(O11) What documentation must a manufacturer, importer, or dealer obtain from
law enforcement officers who purchase semiautomatic assault weapons and large
capacity ammunition feeding devices for official use?
Licensees may transfer semiautomatic assault weapons and large capacity
ammunition feeding devices to law enforcement officers with the following
documentation:
(1) a written statement from the purchasing officer, under penalty of
perjury, stating that the weapon or device is being purchased for use in
performing official duties and that the weapon or device is not being acquired
for personal use or for purposes of transfer or resale; and
(2) a written statement from a supervisor of the purchasing officer,
under penalty of perjury, stating that the purchasing officer is acquiring the
weapon or feeding device for use in official duties, that the weapon or device
is suitable for use in performing official duties, and that the weapon or
device is not being acquired for personal use or for purposes of transfer or
resale. In the case of a transfer of a semiautomatic assault weapon, the
supervisor’s written statement must also state that a records check reveals
that the purchasing officer has no convictions for misdemeanor crimes of
domestic violence.
In the case of semiautomatic assault weapons, licensees are required to
retain the above statements in their permanent records for a period of 5
years.
[27 C.F.R. 178.129, 178.132, 178.134]

(O9) May law enforcement officers keep their semiautomatic assault weapons and
large capacity ammunition feeding devices when they retire or leave their
employment with a law enforcement agency?
No. They may not lawfully keep semiautomatic assault weapons and large
capacity ammunition feeding devices that they purchased or acquired as their
own property. However, the law provides an exception for items that belong to
a law enforcement agency and are transferred by the agency to an officer upon
the officer’s retirement from, or termination of his or her employment with,
the agency. Neither this exception nor the exception for official use permits
officers to retain their own weapons or feeding devices after retiring or
leaving the agency or to acquire additional items. Officers who retire or
leave their employment with a law enforcement agency should transfer assault
weapons and large capacity ammunition feeding devices that are their own
property to a Federal firearms licensee or another qualified officer.
[18 U.S.C. 922(v)(4), (w)(3)]

§ 922(v)(1) It shall be unlawful for a person to manufacture, transfer, or possess a semiautomatic assault weapon.

.....

(4) Paragraph (1) shall not apply to -

(A) the manufacture for, transfer to, or possession by the United States or a department or agency of the United States or a State or a department, agency, or political subdivision of a State, [red]or a transfer to or possession by a law enforcement officer employed by such an entity for purposes of law enforcement (whether on or off duty)[/red];

Quoted:
So, as a police officer, I CAN LEGALLY POSSESS THE WEAPON, even though it's not for duty use?

Quoted:
AND, if my FFL has documentation on-file proving I am who I say I am, which he knows, is that sufficient to transfer the weapon, or does the paperwork HAVE TO BE ON DEPT. LETTERHEAD STATING I AM "AUTHORIZED" TO PURCHASE IT?