From a Swann Galleries auction in 2010:
very rare. An original poster from the sanitation workers’ strike that drew Martin Luther King Jr. to Memphis, Tennessee the first week of April 1968. This poster and one other, “Honor King, End Racism” were carried by the sanitation workers. Later that week, as King walked out onto the terrace at his motel, he was cut down by a shot purportedly fired by James Earl Ray.
via Still Life