Interesting advertising cover with a Shermack coil. This is a cover for Schirmer's Household Series of Music Books on a window envelope with a circular vignette of a lady at the piano. The stamp is one of those Shermack test vending machine coils on, I believe, Scott US #384. Used with a 4/6/11 Grand Central Station, NY mailing mark and a receiving mark on the back of 4/7/11 for London, Ontario Canada.