I found some additional head markings: two different sizes of "F" (a large and small, AMK supplied the large); a "C" with an "H" inside; a "J.M." on the edge (maybe a Home Depot bolt). All were grade 5. No "L5" head markings but that may mean I just don't have any. These 3/8-16 bolts appear to be unique for exhaust manifold use only, no other application. Just for reference, they are 1 1/4 and 2.0 inch long, not fully threaded (1.0 inch thread length).
Jim