The "A" is for AVIANCA, the national airlines of Colombia.In a nutshell
The first airmail service in Colombia was provided by the German company SCADTA (Sociedad Colombo-Alemana de Transportes Aéreos.
In the 1940's SCADTA became AVIANCA (Aerovías Nacionales de Colombia S.A). In 1945 LANSA (Lineas Aereas Nacionales Sociedad Anonima) was formed. Later issues of SCADTA stamps were overprinted L for LANSA and A for AVIANCA (2 sets of each, one with a "plain" L and A and one with a "decorative" L and A. In the 1950's the two airlines merged with AVIANCA being the surviving airline. A set of airmail stamps was overprinted with a small airplane and the word UNIFICADO signifying the merger.
During the period leading up to the merger LANSA or AVIANCA stamps were required in addition to regular postage for airmail service on the airline. That is why you have regular postage and airmail postage on the same cover.
Here is a history of the various airlines in Colombia:http://en.wikipedia.org/wiki/Avianc...80.931940.29
I haven't found a good on-line site for the history of the Colombian Airmail service. When I do I will post it here.FYI:
You also have a Postal Tax stamp on the cover (the little stamp on the right). This tax, at the time, was required and the proceeds were for building new post office in Bogotá, the capital.