Hello Lone, I do not know why they skipped the SK
rating
but I do understand why they skipped
the SI rating, the I in SI looks too much like a 1 (numeral).
I haved worked in the a few fields of facility design and layout, and have worked with building plans,
layout, etc. and the letter 'I' is left out of
column lines used in building plans to eliminate confusion with the number 1, for example, in a row of building column lines on building plan drawings, the columns listed will be A thru H, skip I, then columns J thru Z are listed (if needed).
I have seen the 'I' eliminated in a few fields of engineering and the API being in a lubrication engineering field, my guess is that is why the SI rating is skipped.