I would guess its a manufacturer's stamp. It definitely doesnt symbolize a grade since its metric. Metric usually have the class stamped into the head. Metric class 8.8 pretty much is grade 5, class 10.9 is grade 8 and class 12.9 is above grade 8. Fastenal manufactures a grade 9 which is equivalent to class 12.9. Take your pick of what fits your application. 10.9 is best (better too strong than too weak) 12.9 is typically only socket head screws.
It's a metric bolt, ASTM and SAE marking and grades do not apply to metric bolts, they have their own system as stated in post 2.
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