Stationery post card from a cheka in Estrada street, Madrid, 1937

Stationery post card Edifil 75 Ff  from a "cheka" in Estrada street, Madrid, to the same city. Madrid postmark of June 30, 1937 and text on the back dated on June 27. On the front, mark "SED BREVES EN LAS CONTESTACIONES" and on the back, at the bottom and with pencil "Estrada, 3ª Galería". Extremely rare piece from an unpublished "cheka" (irregular detention center), with its own censor mark, similar to those used in General Porlier and Modelo prisons, also in Madrid.