What is the marking inside the head? Is it an "N" like the one shown?
This is AMK's individual bolt kit #G4352, $1.50. It is also avalible in a set of (6) for $5, item #B-11915.
Its a 5/16-18 x 7/8, 1/2 across the flats, 3/4 conical encapsulated washer, phosphate gray-black color. It has a "lead point" tread start nose tip w/o treads for about first 1/8".