Bolt grade question

I snapped off a 1/2" bolt trying to take off the operators platform of my project. I was puzzled because I was using a 1/2" drive ratchet when it broke and I didn't think that I was applying that much torque when it let go. Looking at the broken part of
the bolt it wasn't partially cracked through or anything. The steel didn't look bad. Maybe a wee bit of rust intrusion around the shank of the bolt. What i did notice was a lack of markings on the bolt head. Noticed the same lack of markings on the
fender mount bolts that I am fighting to get unstuck. Are these grade two bolts I'm dealing with here?

John
 
The way I understand it is this. No marks grade 2. 3 lines on the head grade 5 and if 5 lines on the head grade 8
 
Yes, no marks is grade 2. Here's a chart:

http://www.k-tbolt.com/bolt_chart.html

Note the major difference in the strengths!
 

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