1/4″ top and bottom plates with a 3/8″ lock “mast” which accepts most puck style locks (sold separately).  This is a no-nonsense design which will required power tools, noise or a cutting torch to defeat.  We sell several different puck locks with varying levels of security.  Do call us if you have any questions. Be sure to check out our installation video HERE We offer fleet discount for quantities of 10 or more Ft. Knox Locks purchased.  Made in America.