What year did Herman Miller stop making fiberglass shells?  

Maybe it should help....
Maybe it should help.
Zenith Plastics (1950-53).
Herman Miller (1953-1972).
Reedited by Vitra (fiberglass until 1989 and polypropylene in 2005)
Modernica since 1983 fiberglass.
http://www.dzoom.eu/index.php?p=3&id_dez=Eames

thanks bay
for the useful information

At least until 1974
I have a Herman Miller Black fiberglass arm shell made by Summit Plastics of Kalamazoo MI with a stamp date on it of 1974. It also has the usual paper "patent number label" found on many post- 1960 chairs.

What does that mean about...
What does that mean about the chairs I have then? It looks like they were made on June 23, 1989 according to the sticker. The year I see associated with 1989 is Vitra, but my chair says Herman Miller. Thanks

In the book "modern classic...
In the book "modern classic - the eames plastic chair" Eames Demetrios states that both herman miller and vitra stopped production "in the early 1990s" due to environmental concerns Ray brought up in 1988.
the latest shells i've seen personally were from 1989, like yours.
(edited for typos)

Thank you Claus
Thank you Claus for that piece of info.