18 gauge shotgun with no serial number ...0480

I have a single shot 18 gauge shotgun with no serial number , just two stamped "18" and "0480". took it to my local gunshop and was told it was hand forged and probably from sweeden, made by the community . they also said they thought it worth like $20,000 - $30,000. any idea where or how i can check into getting it appraised to possibly sell.Lurch at 620-262-1580

maybe english