I recently drilled and tapped a 5/16"-18 in a piece of soft steel, then found that the 5/16"-18 bolt I had chosen wouldn't fit (the bolt was slightly too large). When I found that other 5/16"-18 bolts fit just fine, I got curious and eventually tried every 5/16"-18 bolt that I had. I was amazed to find thirteen different sets of letters (and sometimes numbers).
The one bolt marked "AKD" was slightly too big to fit. Of the four marked "HKT", two fit and two did not (too big). All the others fit just fine (though with slightly differing amounts of slop); they were marked "AFE", "AHB", "AKD", "AMD", "BL", "FH", "HBJ", "HBN", "HBS", "HHW", "HKT", "307A CYI", and "307A JZ". A few had no letters at all. I got these bolts at different times and places over an unknown number of years, at various hardware stores and big home centers. They are all steel, not galvanized, not stainless (as far as I know), and all the threads are the same (18). None of the heads bear any of the lines that are supposed to identify the grade.
I've been searching the internet, and I found many sites that claim to explain these codes, but so far none of them include the codes I found on my bolts. Can anyone point me to a comprehensive reference for these codes?