!966 Streamline Prince Mod 19

I've just acquired an older Streamline trailer and was told by the seller it was a 1966. It was a bill of sale only, so there is no title or registration and cannot locate the vin #. There is a tag below the door that says it's serial # is 9031. It has a Calif DOH number and an area on the tag that has M-71. does this mean its actually a 1971 and not a 66? its a 19' Prince and has the high shine to it if that helps to identify the year. I understand 1966 was the last year that they had the high gloss shine. Anyone have any idea on how I can sort this out? Thanks Woodman