A453-660 bolt/nut

[sks4338]

I have this bolt/nut specification.

SBN-FULL THD UNC (ISO) A453-660 HVY-HEX-NUT SEMI-FIN 8UN-ANSI GR-2AB

I think this spec. specifies A453-660 bolt and A453-660 nut.
But I don't seem to know the meaning of "GR-2AB", because theres is
no grade like "GR-2AB" in ASTM A453.

Please let me know the meaning of "GR-2AB".