$100-$225
Yes, Tanfogio is a brand.
The maker was Gamba flli (Gamaba Brothers) made in the city of Gardone, Italy. Yours was made sometime between 1946 when G. Gamba started production, and 1967, when G. Gamba sold his tooling to his brother, Renato.....and the company became Renato Gamba. These were reported to be well made high quality shotguns, but there are so few sales in the US that I cannot find comparable sales to give you a current value- sorry. You can get the best answer by taking your (unloaded) shotgun to a gun show near you, and let some of the dealers and collectors take a hands-on look at it. Condition will drive the value of most guns. There is not a high collector's market for these, but they are considered a quality firearm. Later model Gamba's are offered for sale for several thousand US $$$, depending on the model.
I have a model '66 and the round type is .25.
1873 colt single action army revolver
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.
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
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.
It can be a monumental task if there isn't a obvious makers name stamped. Many companies imported Flli Pietta, Uberti, Pedersoli and others under US store names often with no makers mark on the gun, same goes for the kit guns. Look on the side of the gun or under the barrel or under the lock plate for any clues. Other than that I would look to the on line auctions like gun broker or auction arms, and see if you can spot your gun in there listings.