Thanks Rotunda, that photo makes the marks on the rear of the gun perfectly clear. I think you're right about the muzzle face mark representing the bore size and it is 90mm, but it's marked in centimeters measured from land to land.
Allen,
I don't doubt that N. 570. designates the foundry number, but I do highly doubt that 570 of this model rifled bronze breech loader were ever produced by any foundry in Spain at that time. A foundry number is the manufacturer's own internal control number, and the number stamped on an artillery piece doesn't necessarily represent the actual number in a series of a specific model that were produced by that foundry.
To me the date of 1896 is perhaps the most interesting thing about this bronze cannon; even a few years later at the time of the Spanish-American War, Spain's most current artillery was bought from other countries (Krupp and Hotchkiss). I wonder if they were experimenting with rifled bronze artillery at that late a date in some kind of desperate attempt at producing their own armament? At any rate it shows just how far Spain had slipped backwards as regards it being taken seriously as any kind of military power; remember that the venerable French 75mm was already in service by 1897.
