The crew was tripping out of the hole when the clevis attaching the back-up snub line to the tongs became detached from the tong. The tong swung around striking an employee and he was knocked over by the tongs and to the rig floor. The employee only suffered contusions to his legs and body. Prior to the incident the cotter key for the bolt was lost and replaced with a larger “diaper pin” type safety keeper used on derrick pins. During the TOH the keeper was dislodged from the bolt and the nut vibrated off. The bolt dropped out of the clevis and the tongs were then free to swing around the table.