Hi huff,
I used the handy-dandy new HL Product Search Form http://www.harbourlights.com/search_products/search.htm over on Harbourlights.com (using Keyword: event) to come up with these figures:

1996 Event Piece, Sunken Rock, NY: 8,132 produced.

1997 Event Piece, Edgartown, MA: 8,008 produced.

No numbers have been posted there yet for Roosevelt Island (1998) or Kilauea (1999), but this might give you some idea.

It will be interesting to see how many Kilaueas (especially) were made, since this was a bigger, more expensive piece than the other three Event Pieces had been.

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-Art