I haven't bought graded bolts for ages, but I also remember it being 3 hash marks for Grade 5, and 5 hash marks for Grade 8. I remember it because I thought it was quirky that there was a Grade 5 but that 5 hash marks wasn't for that grade (which, if it had been 6 hash marks, wouldn't have stuck in my mind).

This was all pre-Internet - I guess now I could do a search and find out more about it