Reportedly, 200 Model 1841 rifles, including this example, were altered to the Lindner breech loading system in late 1861 by Allen & Morse of Boston for U.S. service in General Butler's forces, differing from the 100 altered for Massachusetts. Instead of a barrel mounted rear sight, these have a long range rear peep sight mounted on the upper tang/breech cover. The locking "screw-sleeve" on this rifle is marked "PATENTED/MAR. 29, 1859" on top in reference to Edward Lindner's U.S. patent number 23,378. Lindner's breech loading system utilizes a locking "screw-sleeve" coupling that joins the breech end of the barrel to a tip-up threaded breech block with a chamfered face. To load, the knob on the screw-sleeve is rotated counter clockwise using the right hand, which allows the spring loaded breech block to tip upwards for loading with a combustible paper contained cartridge or loose powder and ball. Once loaded, the spring-loaded breech block is pushed down and held with the left thumb, and the screw-sleeve is rotated clockwise gripping the knob with the right hand; creating a tight gas seal. These Lindner conversion rifles have a distinctive reinforcing metal plate, brass on this example, ahead of the trigger guard fixed with a screw. The brass upper tang/breech cover is fitted with a detachable windage adjustable peep sight that would have been mounted at the time of its Lindner alteration. Seven-groove rifling. The breech block is marked "U.S./J.P.C./P" (James P. Chapman, inspector) on top with serial number "15" on the underside. Matching serial number "15" is also marked on the left of the stock hidden beneath the rear barrel band and underneath the brass reinforcing plate. "WINDSOR VT/1850" marked at the tail of the lock and "ROBBINS/&/LAWRENCE/U.S." at the center. Includes a rare Drake pattern socket bayonet marked "B.55" on the collar, a small piece of a broken period leather sling attached to the front sling swivel, tools and an extra nipple in the patchbox. U.S. Lindner alteration rifles are described on pages 181-183 of the book "American Military Shoulder Arms Volume III" by George D. Moller.