I went to a local building supply company - one that tends to have a better selection than the big-box places - seeking some Grade 5 carriage bolts.
They pointed me to bins of galvanized carriage bolts. I checked and did not see the 3 radial lines that normally mark Grade 5. They said "galvanized bolts do not have the marking, but rest assured they are Grade 5"; in fact, I believe they implied that galvanized are always Grade 5 (or higher).
I tend to think they are lying or misinformed, but thought I would seek clarification.