The 531 first appeared in the 1988 catalog as the model 531 Bucklock-1, with a Sawby locking mechanism named after the inventor, Scott Sawby. Approximately 2,000 531's were released. Shortly after release, it was discovered that the locking mechanism failed to hold the blade in the locked position when pressure was applied against the back of the blade (i.e., the spline wack test). This fact, along with Buck's concerns about a lock release that required the fingers to be placed underneath the blade, resulted in cancellation of the model and a recall of the knives that had been issued. Most knives were, in fact, not recalled so they can still be found fairly readily. The 531 was reintroduced, again as the model 531 Bucklock-1, in the 1991 catalog with a mid-body locking action similar to the 110. The 531 lockback was then discontinued in 1994.
Edit to add: The model 531 was used as the very first Buck Collector's Club knife.