Justin- the Peterlongo guns were in general extremely well made high dollar guns-the ones I have seen at auction (and not that many of them) have sold for prices in the thousands of $$$. An accurate assessment of what you have is REALLY going to need a hands-on appraisal by someone that knows these guns. You might contact one of the great gurus of firearms, Jim Supica, and ask for a referral from him. (Google Jim Supica, Old Town Station)
Following is info on the company you may already have:
Johann Peterlongo was an Innsbruck Austria gun maker 1826 to 1898. Richard Mahrholdt & Son, also known as the Tiroler Waffen fabrik Peterlongo, Innsbruck, Austria was founded as Tiroler Waffenfabrik in 1854, and became Mahrholdt & Sohn in 1939. Richard Mahrholdt lived from 1878 to 1949. From 1898 to 1918 he was head of Tiroler Waffenfabrik Peterlongo Richard Mahrholdt & Sohn (Tyrol gun factory P. R. M. & son) in Innsbruck/Austria. That factory was founded by Johann Peterlongo in Innsbruck in 1854. The production of Tiroler Waffenfabrik Peterlongo R. M. & Sohn included • target rifles • rifles for high power cartridges and repeating rifles • drillings • side-by-side rifles • over-and-under rifles with different calibres • over-and-under shotguns • over-and-under rifle-shotguns • scope mounting
R. Mahrholdt was in good contact with the Suhl gunsmith August Schüler. In his time he was known as a good expert of gunnery. He is the author of the "Waffenlexikon", which was published in 1931 and its second edition in 1937. This book is even today in publication and revision.
Mahrholdt was one of the biggest gun gurus of pre-war Germany. He was also a master gunmaker with a safety drilling of his own design. His greatest legacy was the "Waffen-Lexicon", first published in 1931 followed by editions in 1937. It is still in print, but has been heavily edited so that the good info on older makers has been lost. If you can find the 2nd edition, get it. It is a treasure trove of gun lore, patents and company profiles now lost to posterity.
Mahrholdt managed the Peterlongo guns works in Innsbrück, Austria from 1898-1918 after which he took over as his own under the name of Tiroler Waffenfabrik Peterlongo, Richard Mahrholdt und Sohn. After Mahrholdt's death in 1947, it was taken over by his son, Herbert, who was also co-author of later editions of Waffen-Lexikon. The Tiroler Waffenfabrik (Tyrolean Arms Factory) closed shop some time in the 1970’s. Credit to CLUB LITTLEGUN (Photos d'armes de collection ) and Janez Hartman (Slovenia).
But just because I'm a nice guy, and I like to help folks out, I'd be glad to send you a nice, crisp, $100 bill, and a prepaid UPS mailer, and a copy of my FFL- and you can just send that old European shotgun to me, and not have to worry about it anymore.