A2 Stainless Steel M6 U Bolts From U-Bolts-R-Us
Dimensionally reasonably accurate (internal width slightly greater than stated size) - 'legs' are not quite parallel (very slightly divergent - but easily 'squeezed' in a soft-jawed vice if need be) - the quality and 'fit' of the threads is better than some I have seen. Overall, more than adequate for the task (clamping an aluminium pannier frame to the forks of a mountain bike).