My package from NobleKnight arrived & the 12 pg "1981 Information Brochure"
is the 12 pg PreReg book with the Willingham cover. Tucked inside it is a loose half-page insert titled
SPI at Gen Con XIV.
I also got these:
1981 Gen Con East Schedule and Pre-Registration
Format: 10 pages on a continuous single folded sheet
Cover Artist: N/A
Interior Font: Very tiny
Notes: Gygax gave a seminar; Ward ran a Gamma World tournament. Lots of Wilmark Dynasty tournaments.
1983 Gen Con XVI Booklet (Pre-Registration)
Format: 32 pages; magazine-sized
Cover Artist: Timothy Truman
Interior Font: Similar to Dragon magazines of the time period
This seems to be a pre-registration booklet rather than the actual convention booklet (it's shorter, has pre-reg info, has only TSR ads, and looks like it was removed from inside a magazine).