This is a type I Merrill carbine.
Later in the war, a second type was introduced. For economy purposes, the brass patch box was omitted, the steel plunger attached the the breech lever was replaced by a brass plunger to reduce gas leakage and the breech lever latch was altered.
Merrills were widely issued to federal troopers in both the Eastern and Weatern theatres receiving and overall acceptable rating. It served admirably in several of the major battles and campaigns if the Civil War. Some of Union General John Buford's cavalry used Merrill's to hold Confederate General Harry Heth in check on the first day of Gettysburg. Likewise, some of US Grant's cavalry carried Merrills in the Vicksburg campaign. The Merrill was not, however, without complaints. Some regiments complained of the breech lever mechanism breaking or being finicky in combat. Still, the US War Department purchased over 14,000 Merrill carbines.
The Merrill depicted herein is mine. My personal observation of the weapon is that it is very light with a somewhat slight stock. Compared to my Sharps and my Burnside, it is much smaller in dimension and I wonder if it stood up well to the rigors of campaign. I have read of Merrills being found broken at the stock where the firing hand is placed. Looking at it, in comparison to the Sharps, This part of the stock on the Merrill is significantly less robust. Mine has not been broken, but it does show a stress crack in the wood.