No, you didn't miss anything. The triple-alpha SN list in SCSW doesn't include all possible triple-alpha combinations, only those (presumably) that the authors had information on at the time of publishing. The list in the forthcoming 5th Ed. should be more inclusive.
As you may have noticed, most of the AZ* SNs listed were used in 1988, so that's probably your best guess for now. There's a single AZ listed for 1989 (AZJ), but a 1989 gun should be a 15-6.
