Stralsund is a city in Northeast Germany, on the Baltic Sea. The maker's name is Jauer, not the better-know Sauer. The proof marks are S = Smoothbore, W = Choked, and U = Final and definitive proof. The gauge is 16.
Kruppscher means "by Krupp", Prima flussstahl simply means first class billet (poured or "flowing") steel (no it is not Icelandic). Krupp did not make barrels but provided the barrel blanks. I don't know what, if any, agreement they had with the gun makers to put their name on the completed gun; perhaps it was done only because of the high reputation of Krupp steel.
Jim