A. L. is the abbreviation of the Latin phrase "Anno Lucius," meaning "In the year of Light." In Craft (or Blue Lodge) Masonic records you will find 4000 added to the calendar date (2006 is A.L. 6006). This comes from Bishop Ussher's dating of creation on 23 October 4004 B.C. (from his 1650 book, "The Annals of the Old Testament").

More information on Masonic chronology (each "level" uses a different system) can be found at this website:

http://www.mtararat44.org/content/view/44/1

With "597" it looks like you are missing a number...perhaps the stone was partially covered or worn at that spot. If it was 5970, it would be A. D. 1970 (by the way, A. D. is the abbreviation for "Anno Domini" or "In the year of our Lord").