An open clamp structure capable of withstanding very high tightening forces, with a clamping band whose overlapping inner and outer band portions are mechanically locked by a mechanical interconnection including outwardly extending hooks near the free end of the inner band portion adapted to engage with apertures near the free end of the outer band portion; for purposes of tightening the clamp structure upon engagement of at least one hook in the corresponding aperture, a deformable ear with two generally outwardly extending leg portions interconnected by a bridging portion is provided, whereby the bridging portion includes reinforcing means; at least one of the hooks is thereby a cold-deformed hook having force-engaging surface means while another hook is a combined guide and support hook having inclined guide and support surface means formed by its rear edge facing away from the free end of the inner band portion; the reinforcing means is in the form of a generally pan-shaped depression extending over a substantial part of the length and width of the bridging portion.