Anyway, the five digit batch codes had been in use up until 1997/98. With the newer four digit batchcodes, the year of production can be told by the first digit (a number). In the nineties the codes were printed in white/silver, from the year 2000 onwards they were black.
As for the five digit codes, I have one that starts with a 7 and according to the leaflet inside the box it is from 1997. So I assume that in those cases the first digit refers to the year of production as well. If that's true, yours must be from 1989 because in 1999 the four digit codes were already in use.