Indeed, in the style guide there are 5 shades, but if You want to count them, then there's ONE highlight, and You have TWO highlight shades on the sail.
In any case, That there are 5 shades
available doesn't mean that You
have to use them all every time on every larger part of the vessel. In fact, the "dark shadow shade" and "darkest shadow shade" are practically only to show the shade under overhangs or on the parts that are facing "backwards" (relative to the source of light, which, by convention, is in the general direction of upper-right corner of the template.
(in the false-color sample provided in the 4th chapter of Style Guide the "dark shadow shade" and "darkest shadow shade" are also used for railings, ladders etc. as well as to "demarcate angular breaks in the superstructure less than 90 degrees in angle", but in fact number of SB Artists, myself including, use separate shades/colors for these - but that's not the issue discussed here, and now we're talking only about shades of the "basic form" of the hull and sail/superstructure)
Btw. I'd say that the "shadow shade" on the aft part of the sail is unnecessarily wide - although this is one of the things left to Artists' individual choice, in practice, because of the limited amount of shades we use for simplicity, parts that are technically "facing backwards", but just slightly (and are nearly parallel to the general axis of symmetry of the vessel) are not shaded until their "facing backwards" exceeds approx. 45 degrees.
As for the "series I" thing - as far as I know, there was no "Dekabrist series II" (or more) - simply because there were just 6 submarines of the class. Term "Series II" indeed exists in literature, but refers to the follow-on class of submarines, the "Leninets" class, while "Shchuka" class is "Series III" and so forth - but they are not really made to common design, and the term is of rather administrative origin.
And also, emphasis on "series I" could imply that the drawing shows the boat in some sort of "early configuration", whereas You've depicted it in the "modified" configuration".