Bill's collection of Seebeck issues was heavy in essays, proofs and covers since that was a primary interest of his.
Here are some nice El Salvador covers from his collection.
A charming cover from Santa Ana to New York. The 10c registration stamp is not common on cover. The 24c paid the double rate via San Francisco. Note the color of the 24c. It is an orangish-yellow. This is the color of the original. Any other color on the 24c is a reprint!
A double rate cover (2 times 15c per 15g) from Sonsonate to Switzerland paid by the 30 centavos first issue of 1896.