If you look at the manuscript mark on this letter, it is easily read as S Plains, Maine.  The tough part is finding if a South Plains Maine ever existed.  That explains the question mark in the headline.  If anyone can provide more information, please send it along.  Cover is in poor condition with tears around the stamp and through the manuscript mark. Plenty of age wrinkles, too. However, the stamp is fantastic. Right margin copy with all good margins; tending to a little close at top left.  Strong color. Addressed to G. B. Macomber in Rockland Maine.