I am sorry, but I am not sure what gun you are talking about. The following are lists of U.S. Military Rifles and Muskets that would be correct for the date of the lock and Millbury is a town in Conn. rather than a contractor.
On M1814 Rifles:
This was the U.S. Model 1814 rifle designed by Robert T. Wickham.[1] Two manufacturers made this type, Henry Deringer of Philadelphia and R. Johnston of Connecticut
https://en.wikipedia.org/wiki/Model_1814_common_rifle
On M1817 Rifles:
After producing the Model 1814 common rifle through contractors, the military decided to do the same with the Model 1817. The Harper's Ferry Arsenal produced a pattern weapon, which was then taken to gunsmiths to be copied.[2] The rifle was built by Henry Deringer of Philadelphia (13,000 made), Nathan Starr & Co. of Middleton, Conn. (10,200 made), Simeon North of Middleton, Conn. (7,200 made), R. Johnson of Middleton, Conn. (5,000 made), R. & J. D. Johnson of Middleton, Conn. (3,000 made).[1]
https://en.wikipedia.org/wiki/Model_1817_common_rifle
On 1816 Muskets: Contractors
J. Baker - Philadelphia, Pa. - Type I11
P. & E. W. Blake - Old Whitney contract - Type I1 E. Buell - Marlboro, Conn. - Type I, possibly Type 11
A. Carruth - Greenville, S. C. - Type I B. Evans - Philadelphia, Pa. - Type I, 11, and I11
W. L. Evans - Evansburg , Pa., marked "VALLEY FORGE" - Type I and I1
R. & J. D. Johnson - Middletown, Conn. - Type 11 and I11 D. Nippes - Mill Creek, Pa. - Type I11
H. Osborne - Springfield, Mass. - Type I, 11, and I11
L. Pomeroy - Pittsfield, Mass. - Type I, 11, and I11
N. Starr - Middleton, Conn. - Type 11, possibly Type 111
A. Waters -
Millbury, Conn. - Type I, 11, and I11 Eli Whitney - New Haven, Conn. - Type I, 11, and I11 M. T. Whickham - Philadelphia, Pa. - Type I and I1
From: NOTES ON THE MODEL 1816 U.S. FLINTLOCK MUSKET
By R. N. Kennedy, Jr.
Gus