See JohnZ's comments in this thread: http://www.camaros.net/forums/showthread.php?t=97516&highlight=bolt+head+markings

Hmmm. Nevermind. I see now that you were an active particpant in that thread.

 

The GM part number is 3828782. 3/8-16 x 1 1/4, 9/16AF hex flange, grade 5 bolts.
Not sure exactly what info is going to be handy to you, but bolts with an "A" in the center of the head (AMK # B-10084), an "M" in the center (B-10083), or an "F P" around the grade markings (B-10082) are available if you need replacements. $10 buys 8. All are listed in the catalog with that GM number and the application being "Chevy Bell Housing & BB Intake".
www.amkproducts.com

 

Thanks Steve, Yes i checked them out in AMK. I have also believed that the M,s were correct for years, as a matter of fact, we used to install M,s in our restos. ( worked at a corvette/camaro resto shop for 17 years)
I believe my bolts to be original, have owned this Z28 since 73. The EWF bolts are real nice shape, they just got me wondering? and i am allways eager to learn.

 

I wonder why the differences in the first place..
Maybe Flint and Tanawanda used different bolts to each other.
Maybe the differently marked bolts were from different suppliers and they swapped between them depending on availability and/or cost.

All being the same part number, I'd find it very surprising if the applications were different.