goofy bolt headstamps

How common is this- inch threaded fasteners with metric grades stamped on the head?
Came across some "12.9" stamped allen head bolts with a real 5/16-18
thread. Does anybody in Taiwan ever stamp grade the grade 5 or grade 8 lines on metric fasteners?
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On Sat, 9 Mar 2019 02:19:11 +0000 (UTC), Cydrome Leader

A class 12.9 bolt is described as being "alloy steel quenched and tempered" and has a minimum tensile strength of 1220 MPa, which is about 176,946 psi.
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John B.
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On Sat, 9 Mar 2019 02:19:11 +0000 (UTC), Cydrome Leader

12.9 grade steel means that a component manufactured form this grade of steel has 12X100 N/mm^2 tensile strength and 9 is a multiplier means 90% of 1200 = 1080 N/mm^2 is the yield strength of the grade. Similarly we also have grade 8.8 , 10.9 , 6.8 also.
Not COMMON, particulary in non-metric countries, but I imagine not uncommon in "metric" countries where an SAE threaded bolt is required.
Much more "descriptive" than "grade 3, Grade 5, or Grade 8"
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