Hi Don,
As I understand it, GM used a few contractors to make the nuts so the face marking could vary. I know you asked for a '69 but I did check my '67' which have the original nuts, and found the following:
67 04B LOS - waffle pattern on face - remains of zinc plating on all nuts
67 05B NOR - DMC stamped on the face - zinc plating on all nuts
I would think zinc would be used for a fastener like this.
Mike