Puerto Rico 1938-40 US possesions Semi-official local airmail stamps Partial sets MH 17777

Puerto Rico's semi-official airmail stamps were issued between 1938 and 1940 by Aerovias Nacionales (Aerovias Puerto Rico Inc.)  to cover express and air transit fees on the island. 
Because they were considered "cinderellas" (labels rather than official government postage)  
they were initially prohibited by the US Post Office but later tolerated as an additional optional air charge.

1938 Issue: Sanabria 501 503-7 Issued 10.000 stamps
Aerovias Nacionales issued six values initially along with two surcharges to pay for express air delivery. 
These stamps are sometimes documented in specialized catalogs under the Sanabria system as S1 through S8.
1941 Issue: Sanabria 508-11 Issued 10.000 stamps
A striking set of eight bicolored triangle stamps was released. 
However the US Post Office Department firmly prohibited their use on September 16 1941  
making them highly sought-after by postal historians and airmail collectors.