There are no known records from the Penn Yan Company, so there's no way to connect the serial number to a build record.
Some information on Penn Yan can be found by using the "search" function above (and typing in "Penn Yan" and maybe adding Rainbow) and by going to www.dragonflycanoe.com/id/. Scroll to Penn Yan on the left.
After WWII and before 1957, Penn Yan used the first 2 digits to indicate the year of manufacture. A serial number with the digits starting with 51 probably indicates that the canoe was made in 1951. Gil