The grade system for nuts is a number that represents 1/10 of the specified proof load stress (in kgf/mm2). The proof load stress corresponds to the minimum tensile strength of the highest grade bolt that can be used with that nut.
Although it is common for Grade 5 bolts to feature three slash marks and for Grade 8 bolts to feature six slash marks for identification purposes, it’s important to note that not all Grade 5 or Grade 8 bolts will feature these marks! Quite often, specialty bolts (made for an automaker’s specific requirements) may not feature any marks at all, or may feature a unique symbol. If you’re performing a restoration, it’s best to stick with factory-original bolts where grade may be in question. Otherwise, if the bolt head isn’t marked (blank head), and if you don’t know where the bolt came from, you should assume that it features a hardness grade of less than 5 and shouldn’t be used. If the bolt was produced by a known quality maker, such as ARP, you can be sure that the bolt is correct for the application recommended by the maker. I’m simply saying that if you don’t know where a bolt came from, and it’s not marked, throw it away and buy known-quality bolts instead.