the keeper 30 engages the underside of mat b or t the contact with the mat b or t prevents removal of the pin assembly 10 from the mat an additional quarter turn or 90 degree rotation clockwise or counterclockwise of rod 40 correspondingly moves the keeper 30 a quarter turn or 90 degrees and the keeper 30 rotates back to its rest position with the entire volume of keeper 30 within the longitudinal projected volume of pin body 10 a during rotation of rod 40 as shown in fig2 the second tapered edge 32 of keeper 30 first contacts the bottom of mats b and t and when the first and second crown 38 and 39 of the keeper 30 contact the bottom of mats b and t as shown in fig9 the first and second crowns 38 and 39 of the keeper 30 in combination with enlarged head 12 provide a fastening force on the mats t and b thereby securing the two mats t and b together referring to fig4 the cylindrical shaft 40 b of rod 40 is shown fitted into central longitudinal bore 20 with central longitudinal bore 20 shaped to snugly surround shaft 40 b to allow rotary movement of rod 40 relative to pin.