Probably .22WCF or what the Europeans would call 5.35R "Vierling"(?). A good small game caliber in its day; comparable to our .22WRM ballistics. (Ned Roberts thought it was a deer and medium-sized varmint ctg; on the other hand, he REALLY could shoot!)
Hope the bore is still good; many of these got fairly badly wiped out by not cleaning after using black powder and corrosive primers or shooting too many Hornet jacketed bullets in them. Altho it looks to me like whoever owned that one knew how to treat a fine firearm!
