Cover franked with 1927 20c pink (SG 294) tied by superb strike of undated 'P.M. No. 4 CORREO POZO AZUL 'Negative Seal' cancel in black of the Paraguayan Military P.O. at Pozo Azul. Addressed to ASUNCION. Likely to be a strike made by favour after the war, however still a scarce cancel.