There are three different versions at work here. The Battletech Compendium by Jerry Stenson came out in September 1990. It was a softcover release with the ISBN 0555601286. It was listed as FASA 1640. The Battletech Compendium: Rule of Warfare was a hardcover September 1994 release. It has an ISBN of 1555602479 and a FASA number of 1691. A softcover The Battletech Compendium: Rule of Warfare was released in September 1995. It has the ISBN of 1555602916 but also has the FASA number of 1691. The last two are combined together.