You will not find these interesting issues listed in Stanley Gibbons or Scott catalogues!
In August 1925 three new air routes connecting Cochabamba with Sucre, Oruro and La Paz were inaugurated by the South American Junkers Mission under a contract with the Bolivian government. The Cochabamba to Sucre first flight took place on 5 August (59 covers flown), the Cochabamba to Oruro on 11 August (65 covers flown) and the Cochabamba to La Paz flight on 14 August (134 covers flown). The aircraft used was an F13 Junkers monoplane that had been carried by rail in crates to Cochabamba where it was assembled.
For each of these first flight events a specially overprinted stamp was produced and authorized by the postmaster and the prefect of the State of Cochabamba. The current 50c orange Coat of Arms postage stamp was typographically overprinted with one of the three types of three-line overprint for each flight – “CORREO AEREO / A SUCRE / 5-8-1925” (in green, 400 produced), “CORREO AEREO / A ORURO / 11-8-1925” (in green, 300 produced) and “CORREO AEREO / A LA PAZ / 14-8-1925” (in carmine, 1000 produced). The overprinting was applied locally by the Severo Cuenca printworks at Cochabamba. Unusually a small plate of ten overprints were made and applied to horizontal strips of 10 stamps, so no vertical strips or blocks can exist. The stamps were sold at the post office with a surcharge of 10c to defray printing expenses and were valid only on the three inaugural flights.
For one of the inaugural return flights, Sucre to Cochabamba on 7 August, a large three-line handstamp "CORREO AEREO / A COCHABAMBA / 7-VII-1925" was prepared and applied to the stamps after they were affixed onto covers.