Mike Spencer wrote:

Flat head hex socket cap screws, 1/2" coarse thread, black finish.

What, if anything, should the marking on the head, "Y D" tell me about
the grade of the screw?

(Not old bolts; bought new circa 2004.)

Attaching a die insert to the upper die of a 300# air hammer.
Potentially dangerous should a bolt break during operation. Looking
for max strength fasteners. Wasn't so important for the purpose for
which I originally bought the screws.

Ying Dong noodle factory perhaps? If it's safety critical just go spend
the $20 and buy new grade 9 hardware from a reliable source.