The "8.8" has nothing to do with what size it is as it relates to the tensile strength of the bolt.
On bolt heads you can get all sorts of stampings on them including: a manufacturers mark, tensile strength, what grade they are ("A270" stainless steel for example) or various marks/symbols to indicate all of the above etc.
Link: https://www.google.co.uk/search?q=markings+on+a+bolt+head&rlz=1C2TEUA_enGB574GB574&biw=1366&bih=643&site=webhp&tbm=isch&tbo=u&source=univ&sa=X&ved=0ahUKEwjS_Z_spOLNAhXIBcAKHW6bCFMQsAQIIg#imgrc=_
Although there can be some variants on the head size (spanner needed) depending on the country of origin.
More info: http://stainlessautomotivefastenings.co.uk/pdfs/HEXAGONHEADIDENTIFIER.pdf