HP 71B funky serial number

[polbit]

I've never paid attentions to serial numbers, but I just picked up another 71B that has a serial number that starts with 9999A. Now my understanding is that the first digits are YYWW, and YY is number of years from 1960. Well, I either have a 71B from the future, some sort of prototype (it's ROM 1BBBB though), or I'm not understanding the serial number convention... Am I missing something obvious here?

Thanks!