So your saying in 1992 they (all GM) began calling the 91 engine " Series I " ?
My GM parts book clearly shows they (GM) called the 92 H body engine "L27" Its written like this - 1992 H ENGINE ASM-3.8L V6 PART_ 3.8L(L27)
Series I & II should only apply to the L67 as there is clearly 2 versions of this engine. Shop slang has transfered this Series I & II thing over to the NA engine with the same dates apparently. This however does not change the fact that no significient change occured with the 1991 L27 to classify it as the first of a Series. 1988 and 1996 were the significient years for new engine design for the NA engine.