They began to be issued in 1855 when they replaced the 1851 version of the frock coat among the mounted regiments and mounted artillery batteries. Like Rusk County the Mounted Rifle Regiment, along with the four other mounted regiments, was converted to a "general porpoise" Cavalry In 1861.
The 1857 U.S. Army regulations covers the uniform article LI (51) on pages 438-439. Although the 1861 Regulations a better description of the mounted jacket.