Here's a tough keg !! -- General Porcelain only marked a small portion of their production runs so any insulator marked G.P. Co. is considered uncommon. There are tons of unmarked U-61 kegs but not nearly as many marked kegs. Of the three incused markings known this is the one with periods and no oval. Has two flat slices to center ring to right of incuse marking and a flat slice to rear lower skirt. Use "view original". $10.00 + shipping. First email that says' "I'll take it.". No PayPal.