What does the .O. L.9. on the muzzle face signify?
As close as I can tell being a gringo non-Espanol speaker, it is the gun designation, exactly or approximately:
Obus Largo, 9 (Spanish inches=Pulgadas).
Obus Largo means Long Howitzer, I think, someone correct me. However, when there are short and long versions of a particular Spanish piece of a given caliber, as there usually were, they are described as either short or long, by "Corto" or "Largo" respectively. My Spanish short 8 pounder gun has on the muzzle, as I recall, "8 Co."
Spanish were pretty much like everyone else back then, round solid-shot-firing guns designated by "pounder" and hollow shell-firing howitzers and mortars designated by bore diameter.