Suzuki, at least as recently as 2008, has provided a table of standard bolt torques in their motorcycle service manuals that refer to metric bolt head markings. Two values are listed for each bolt size. The first is for Conventional or "4" marked bolt[s] and the second for "7" marked bolt[s]. The table includes figures showing bolts either unmarked or with a large 4 or 7 on the head.

I am aware of ISO strength grade markings like 8.8 or 10.9 but cannot find any information that correlates with the Suzuki markings. Is this a Suzuki or Japanese system or what?