Thanks Pernod. That code isn't quite doing what they seem to think. CMD10 returns a CRC7 at the end, not a CRC16, so they're actually going by the CRC in 1 byte and the month and low 4 bits of the year the card was manufactured. (The CRC7 includes the manufacturing date, so it's a little redundant, but I can't imagine people are swapping cards that much).
I'll probably use the file date on the image file as the manufacture date and calculate the correct CRC7.