Just for the record:

The vendor says the "Y D" markings on flat head, hex socket cap screws
are maker's mark (maybe even, as suggested here, Ying Dong noodle
factory :-) unrelated to the grade of the cap screws.

The grade is ASTM A574, comparable to (better than?) SAE grade 8.

For my purposes, flat head is preferable because I can pre-load the
joint between two parts subject to impact, eliminating risk that the
impact force would fall on the fasteners.

Tnx for the replies,
--
Mike Spencer Nova Scotia, Canada