I'm going to take another guess at the mark I called the "R with horns."
Old Spanish bronze cannons still in use during the 19th C., and those newly-cast during that period, are often found with stamped characters on the upper muzzle-face which are abbreviations for their type and caliber. Some examples of Spanish upper-muzzle face marks I've seen:
9" howitzer, long (cast in 19th C.) "O.L. 9"
8 pounder, short, cast in 1808 "8 C"
12cm mountain howitzer cast in 1844 "O.12.M.L." (photo)
16 cm bronze gun cast in 1795, later rifled "16 C"
In looking at the Pedrero's mark, it looks more like a "P" with some little arms on it, not stamped cleanly like the "P." Maybe the little legs or arms were added later as somebody's form of graffiti.