DK55 or DK551. Manufacture date question

[kck8385]

A couple questions regarding the attached pictures.

First Picture: Sticker is located on the right side of the instrument panel and states this tractor is a DK551 with a S# starting with an F.

Second Picture: Sticker is located on the inside of the open station behind the seat.

Questions:
1. What is the difference in a Model DK55 and DK551?
2. Am I correct that the F prefix in the S# means a 2006 manufacture date?
3. In the second picture what does the S# mean? It is different from the other sticker.

Thanks,

Kevin