FIG. 1 is a top plan view of the strap buckle assembly showing my design. FIG. 2 is a side elevation view thereof, an opposite side view being identical; FIG. 3 is an upper perspective view thereof; and, FIG. 4 is another upper perspective view thereof, showing an extended position of the strap. The broken lines are presented for illustrative purposes to indicate continuing length of the strap of the design.