Without seeing the markings it is impossible to answer precisely. It is likely that you are looking at proof marks. European nations require that firearms be submitted to a government testing facility (a Proof House) where firearms are tested by being proof fired with a high pressure test load. Guns that have been "proofed" are stamped with a symbol to show that. Each nation has different symbols for different types of guns. Research the term "Italian firearm proof marks" to see examples.
100-1000 or more depending on specifics
Flli Peitta is an Italian gun maker specializing in reproduction black powder and single action guns. Imported in the US by several retailers and distributors.
It was made for marking trees
Gunsmiths, gun collectors, gun-oriented internet forums, you might find someone at the gun show who'll know something about it.
You might find them in the Blue Book of Gun Values.
The compressed air gun, which shot metal bb's was invented before the paint marking gun.
Actually, it is not a real gun. That marking was used by Denix on replica stage prop blank shooting copies of a real gun.
FIE imported various arms made by various makers.
Could be part of the caliber marking
Blue Book of Gun Values would be one place to start
Paintball guns were widly used as marking guns by the wildlife police.
Dixie Gun Works has schematics on there website, if you're looking for parts. If you're looking for take down or cleaning/loading etc. try http://www.thehighroad.org/archive/index.php/t-151485.html