Thanks for the reply Barry! Do you manage the Hamtramck Registry? I just filed my info with you a while back.
The bolt is a 5/16-18 thread, is 1 1/4 inches long including the head and has a washer that is about 7/8". The head has only 3 faint hash marks at the 12, 3 and 9 o'clock positions and three concentric circles. Here is a photo of the head.