The "D" at the end of the HIN is part of a code system used by some manufactures to indicate the month. A= August, B=Sept,...., D=Nov,... L=July. This issue came up for me a few years back when the WA DMV discovered a discrepancy in my Ericson 25+ HIN number and wanted me to apply for a completely new number. As it turned out my vessel was manufacture in April (= I), and at some point in time that letter was changed to the number 1 in the state's records, which didn't match any known valid HIN formats. It was a pain, but I finally got it straightened out.