Product Description UPVC Window Restrictor Hook. Child Lock Restrictor Safety Catch. This essential child safety device is designed to limit opening of a window to 100 mm. It can be released to facilitate full opening of the window, it automatically re-engages restricted position on closing of the window. This product is manufactured from stainless steel and complies to BS6375 : Part 2 : 1987. Easy fit onto window (screws provided).