As mentioned in an earlier post, Bill Welch was especially fond of Essays, Proofs and Covers. Here is a charming postal stationery place proof and cover.
A plate proof on India paper showing the plate layout for the 1890 3 centavos single and message & reply postal cards. Bill had these in several different colors, although none are very common.
A lovely 1893 cover from San Salvador to Cincinnati, Ohio. The 25c value is not common on cover. These stamps paid 4 times the postage rate (11c per 15g), registration (10c) and Acknowledgement of Receipt (5c) with an 8c overpayment.