This three stamps all depict some vessel of which till so far not one has been identified, the stamps depict two ports in the formerly Spanish Sahara, and one in the Spanish enclave Ifni, all are now under Morocco control.
The 1.50p stamp shows the pier at Aaiun in June 1967, she is the largest city in the Western Sahara and now named El Aaiùn.
The 4p stamp shows the harbour of Villa-Cisneros in 1967, which is now known as Dakhla.
The 1.50p stamp shows the island pier near Puerto de Sidi Ifni, a former Spanish enclave, which now belongs to Morocco.
The three ports were built by Grupo de Puertos de Guipuzcoa headed by Ramon Iribarren Cavanilles an engineer of roads, cannels and ports.
The stamp of Ifni was the most intriguing work, which solved the problem to bring the ship inside the port. An island pier was constructed off the coast, on which the vessel berthed. The island was connected by the shore via a cable on which a cable car moved; below the cabin was a hanging basket in which the cargo was transported, as seen on the stamp.
The island pier was in use till a few years after Morocco took control of the enclave, when they built a small port, what has many problems with silting which give reason that the island pier at all was not a bad solution for this port.
The port is now mostly used by small coastal fishing vessels.
Spanish Sahara 1967 1.50p and 4p sg?, scott?
Source: http://www.ingeba.org/liburua/sellos/sel6670.htm and various other web-sites.