Serial no. 5991000 maybe wrong number (not officially exist).
If I'm not wrong! The A1 lower receivers made during 1980~1989 number is up to 54XXXXX. Then start with 6XXXXXX for the A2 lower receivers!
Also if I am not wrong there are only 50 numbers of MK18 MOD 0 officially made for the Gulf War!
So if custom made MK18 MOD 0 serial no. maybe best start with 54XXX01 to 54XXX50!
