The Scofield Bible Commentary, by Cyrus Ingerson Scofield, , at sacred-texts.com
That is, acacia.
Silver thus receives its symbolic meaning -- redemption. The sockets were made of the redemption money of the children of Israel.
Compare (Exo 26:19); (Exo 30:13-16); (Num 3:44-51).
(See Scofield) - (Exo 26:19).