From the looks it goes GS Xmen #1 - to last issue before we went Movie. Then #1-67 in those extra books only (but Stan only wrote the first few issues, his brother wrote the rest he is created has writer see Kevin Smith interview where he got Stan to admit he didn't write the comic)
Got an email as well this week "Thank you for supporting X-Men: The Essential Collection. The limited four-issue run has now come to an end, and we will be looking at the level of interest before making a decision about whether the collection will be launched nationally in the near future."
Received nothing yet but from what i can make out 60 issues are listed with 1991 X Men Volume 2 appears to be number 48 so allot from 1980s