First, on the "PA" head marking, from what I know that just designates a manufacturer and has nothing to do with grading. If there are other markings, like lines on the head, those would indicate a specific grade of bolt.
Scouring through a 1939-41 GMC Master Parts Book for Models 100-460, there is no specific individual hardware part number to dimensions listing.
Knowing that Chevy and GMC shared a lot of the same designs and hardware uses (and part numbers), I looked in the earlier Chevy Master Parts Books for similarly sized hardware and tried to compare between brands to locate a similar sized bolt and where it's listed for use.
The only thing I found that seems possibly similarly sized in the front end/engine compartment vicinity for the 1-1/2 ton trucks is for the front springs "spring retaining clips". The bolts (I believe the GMC book calls for 8) spans the width of the front spring to secure the clips to the spring packs. I believe there are 4 spring pack clips for each side.
Perhaps these came from the front springs???
