On Sat, 9 Mar 2019 02:19:11 +0000 (UTC), Cydrome Leader
wrote:

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?

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"