RPGGeek lists two printings for the original Fiend Folio. So as to identify different printings, what are the observable differences (if any) between the first and second printing?
According to the Acaeum, the 2nd printing has both the ISBN and TSR product number on spine & lower left corner of back cover, while 1st has only ISBN in both places. Instead of the TSR product number on the back, the 1st printing has copyright information: "1981 TSR Hobbies, Inc. All Rights Reserved." and "The designation 'TM' is used to designate trademarks owned by TSR Hobbies, Inc." Internally, the printings should be identical.