According to page 140 of Gerald Posers book "Case Closed" (Paperback edition), it states:
"But what of Oswald's stamp "544 Camp Street" on some of his Fair Play for Cuba leaflets?
On the next page, he again uses "leaflets" plural. The 544 Camp Street address was on one "The Crime Against Cuba" pamphlet but that was a pamphlet, as in it had pages, and not a leaflet or a handbill which usually are only one page.