I never knew that myself. I just checked my registration for my '05 1200S and it says "GSF1200ZK5". I wonder if the Z is a typo.
Most likely, it's a typo of omission.
If your '05 is the white & blue special edition with a fairing, it's a GSF1200SZK5. If it's a naked special edition, it would be a GSF1200ZK5.
The Z is a designator for the special edition paint.