Window sealant
 

"and bedded the windows in black polysulfide (same as the
original)"

In the 13 page letter from S2, they say that black silicone was used,
NOT polysulfide, due to thermal expansion rates of the fiberglass and
plexiglass. To view the letter, go he
http://www.s291.com/public_html/history-referral.htm