International Brigades, two covers to Holland, one through the French post

Cover sent by the brigadist Fritz Denters to his mother in Holland, G.W. Denters-Stallmann, with Barcelona meter dated on July 21, 1938 Heller 8, and censor Heller 5a on the back. The second cover was sent by the same brigadist to his mother in Holand again, circulated with French meter from Rue Bleue, Paris, through the special postal service provided by the Republican government to disguise the Spanish origin of the brigadist's correspondence coming from countires considered hostile to the Republic, and avoid possible retaliation agaist the addressees (See Heller, Ernst:"La historia y el servicio postal de las Brigadas Internacionales", pages 185-192, for covers of the same brigadist circulated throught the same channel). Both covers are an exceptionally rare pair.