I'd say 1987 and someone changed it to a Z for whatever reason, or they didn't have a 2 stamp and used a Z ??
the only thing that makes me think that its "missing" is I was told, idk if its true or not but, the polish SN is, first four digits are the year, followed by two letters, and five numbers after that. that being said the only one that's not accounted for is one of the year number. is this info correct?