The climbing industry standards for bolts in North America is the 3/8-16 bolt.
The flat head is the last item on this page. It is not photographed. It is only used for the 25 Assorted Smalls only. The rest are socket head varying in lengths.
3/8-16 x 1.5" Allan head bolt
3/8-16 x 2.0" Allan head bolt
3/8-16 x 2.5" Allan head bolt
3/8-16 x 3.0" Allan head bolt
3/8-16 x 3.5" Allan head bolt
3/8-16 x 4.0" Allan head bolt
3/8-16 x 5.0" Allan head bolt
3/8-16 x 7.0" Allan head bolt
3/8-16 x 1.75" Flat head bolt