The Republic of Liberia issues 10 different Princess Diana $20 coins in 1997. All were 0.9990 silver with 1.0027 ASW. The one that matches yours the closest is KM# 417, listed as "Princess Dianna in Memorium" but with a reverse of "Portrait and Dates"
Ok, here is KM# 417 http://cgi.ebay.co.uk/1997-LIBERIA-SILVER-20-Princess-Diana-Memoriam-/300422238157