Robert Hughes, 22 Moland St. Birmingham, rapidly expanded his business during the ACW. Some of his marks are found on CSA contracted guns. The marks on your gun are the same as found on other Hughes' guns.
Another example was just posted on Stamped double crowns on 1853 Enfield thread. OOPS, my bad.. it was your post.
Reposted because I still haven't found another example. Highly doubtful it's a unicorn. Maybe someone has one similar but can't make out the lettering.
Just went through my Enfields. I've got a few of the "anchor/RH" P53's. Although not a unicorn, it wasn't a mark that Hughes often used. The location of the stamp on the wrist of the stock gets a lot of wear.
Here's the best one of mine on a Georgia "G" numbered P53, behind the trigger plate as is yours.
Thank you Lanyard Puller. Glad to see other R. Hughes rifles have survived the times. Was there any sign of there once being a cartouche on the stock? I feel there should be more information available about the makers of Birmingham who made these fine rifles. With the four of July just around the corner, I might harass the neighborhood by letting the old girl bark once again.