I don't think it's worth placing any value on the head markings. There's an image in the linked post showing bolts with the same markings, yet are different tensiles. Unless, as you both point out, surface coating/lubrication matters enough - from the **stock-coatings** of these Toyota bolts...
I don't think tensiles are as simple as lower equals weaker - it's a complex and debatable topic. I believe the higher the tensile, the more critical torque accuracy is, the less room for error (tends to snap), whilst lower tends to bend. But it's a debate for people with more knowledge than me...
