Rod, having had many years selling and making fasteners and doing research of fastener manufacturers, I can tell you the following: “TR” stands for Towne-Robinson, a Detroit bolt maker. This firm went out of business in the 80’s. “M” indicates Michigan Bolt who has morphed into another company and doesn’t use the “M” as their mark any longer. The “AP” name has eluded me but I was once told it stood for Allied Products, not sure. I made this fastener while I owned Paragon Reproductions from 1970 thru 2002, when I sold the company. I think the “J” stands for Jackson who is out of business as well. Most US made fastener companies are gone now as fasteners are cheaper to produce elsewhere, to bad about this fact. As you might now see, all fastener companies had to identify their product with their assigned logo so if problems arose GM could track the specific bolt back to them.