Sea Dance comes in three colours: Sable (likely your weight) was first issued in 1989 - and G was the letter used to denote manufacture in 1989; the number (18406) would be the running number of unlimited weights made that year. Do a search on
Scotland's Glass to see images of all three versions.
With engraved weights you often find initials of the engraver - but Sea Dance is not an engraved weight ... would it be the maker? Whilst it was designed by Colin Terris, there will have been many people actually making the weights. I cannot remember having seen this before - has anyone else?
On the other hand, to me the DE looks a bit amateurish - so possibly it was not a standard feature, but someone thought her/his initials should be there.