P=have O=don’t have it
Known as Rio de Oro before 1924, the Spanish Sahara was a province of Spain until 31 December 1975.
When Spanish troops were withdrawn on 26 February 1976, the country was divided between Morocco and Mauritania; the border was agreed on 14 April 1976. Morocco had made her claim by the peaceful 'Green March' in November 1975. A so-called 'Saharan Arab People's Republic' was declared in opposition by Algerian-backed Rio de Oro guerrillas. Existing postal arrangements are not known.
Inside #B55-6: Stamp on Envelope
(Thanks to Attilio for the scan)
Inside #187: Stamp on Envelope