Unbranded Japan U-4 Cast Iron Skillet Pat. Pend... 10015

Mystery maker vintage Japanese cast iron skillets... circa 1950's?  These pans generally have a U- with various numbers, in this case U-4. This pan is stamped PAT. PEND... most are stamped PAT. 10015. I will speculate the number 10015 is a patent number and not a pattern number. This is a pretty well made pan.