Mystery Solved! Unique class 4, 5, 6, 7 bolt mark

Metric or imperial? ASTM, DIN, JIS, other? What does this both head 4, bolt head 5, bolt head 6, or bolt head 7 marking mean? It took me probably 6 hours of investigation time to finally stumble upon what this bolt marking means. Machinery Handbook was silent. Google searches are normally of minimal help, other than tiny pieces of information or pictures. It wasn't until I stumbled upon the proper vintage of Japanese Industrial Standard did I finally discover and verify from the source how these metric JIS bolts were classified, compared to the commonly recognized metric class 4.6, 4.8, 5.6, 5.8, 6.8, 8.8, 10.9, or 12.9. I hope you enjoy this investigation as much as I did. I also encourage checking out my other videos of similar topic including: What washers do within a nut and bolt joint:    • What washers do within a bolt and nut joint.   How to identify a metric or imperial bolt quickly:    • How to identify a metric or imperial bolt ...   How to find bolt or nut size, metric or imperial:    • How to find bolt & nut size, metric or imp...   #hardware #garage #learning #mechanic #diy #lifelessons Disclaimer: of course, this video is for entertainment purposes only........you are responsible for any and all of your own actions......not me. Disappointing we have to say things like this in today's world, but here we are. Stay curious, friends.