Q. "There are letters and/or numbers after the model number inside my Kohler toilet tank - what do those mean?"
A. Kohler offers many variations of almost all their toilet models, and has experimented in the past with many different features. Typically, numbers (or a single letter/number combination like K4 or Y2) after the 4- or 5-digit model number indicate the color code for your toilet. If your model number is followed by letters, those usually have different meanings to indicate the features of your toilet or will indicate that you have a specialty Artist Edition toilet.
http://www.us.kohler.com/onlinecatalog/pdf/105615_2.pdf on page 6 of the pdf directions is how to properly adjust the water level and timing of the bowl water level adjustment. Note the 50psi comment, if it is above 50psi it needs to be timed...If you are at 50psi or below then it is only adjusted by the seal screw. This may be why you are not noticing a difference with the timing screw. So avoid using the high pressure adjustment as you will not gain from adjusting the screw.