Does anyone know the silver content of this coin, not in catalog.

 There are only two coins on this page 

https://www.ngccoin.com/price-guide/world/search/1/?country=LIBERIA®ion=All+Regions&denom=20+Dollars&date=2005&catalogInitials=&catalogNumber= 

 but both are listed as 

Fineness: 0.9990 

 if that helps. 

How bizarre that Liberia would be OK with striking a coin in its name that honored the continuation of slavery in the U.S.

True, but there are several that commemorate the abolishment and struggles of slavery, ie , uncle toms cabin, emancipation proclamation, underground railroad, and Abraham Lincolns assassination are just a few.