The ornamental design for a strapadjustmentbuckle for helmets, as shown and described.
FIG. 1 is a rear perspective view of a strap adjustment buckle for helmets
showing my design, the straps being shown in broken lines for illustrative
purposes only;
FIG. 2 is a front elevational view thereof;
FIG. 3 is an end elevational view taken from the left side of FIG. 2;