The specialist barrelmaker WK = Wilhelm Kelber supplied many Suhl gunmakers, Schüler just one of them, with barrels he bored and rifled.
If the rifle ever passed Mahrholdt's shop, he would certainly have had the "Peterlongo" address inscribed on the barrel. A friendship of Schüler and Mahrholdt is not important here. Another close friend of both R.Schüler and Mahrhold was Karl Burgsmüller, Kreiensen, Germany. He sold many rifles made by Schüler through his worldwide mailorder business. But any other gunshop in Europe may have retailed the rifle as well. So forget your idea fissa about Peterlongo. At least, Schüler, Suhl, is an even better provenance than the Peterlongo, Innsbruck, gunshop. Mahrholdt had most of "his" guns by various gunmakers in Suhl, Zella – Mehlis and Ferlach.