Found the book last night but no go on the 3S suffix. . There's a few others I will post later as they're worth noting but are small block engines including L79's. .
I forgot to note yesterday that 4W is also known and is '68 Chevelle L34 Auto. .
Considering these known Mk1V numeric style suffixes are all for '68 cars (besides the numeric Marine ones) I think this 3S suffix may be something worth investigating further. .
Lastly I just had another thought, aren't the '68 Gibb Novas w/ the L78 M40 a unique suffix that's similar to this....I don't have it in front of me but isn't it a 3 something....anyone? .
~ Pete