I would appreciate if the experts here can shed some light on what these El Salvador stamps are, and what sort of collector value the stamps have in today's marketplace. Someone said these are proofs and hence could potentially be worth much.
Note after Scott #421 says the "S" overprints preceded the 1915 overprints with no evidence they were ever placed in use. The 1915 basic stamp is different from the earlier bicolor with black printing, as it has a mottled dot pattern (underprint? overprint?), probably as a security feature. I've never seen a complete set of "S" overprints.