Here are 2 examples of what I believe to be a local post issue release or some sort of Club issue.The example not overprinted is one of six in various colors all with the same 50+10. There are only 5 of the overprinted variety minus the green blue from the first group. As always any and all feedback is appreciated.
They are listed in the Sanabria World Airmail Catalog as "semi-official", or private issue. The original set of 6 stamps without the overprint was released July 7, 1951 to commemorate the 50th anniversary of the Royal Aero Club of Belgium. There were 6,000 sets printed.
A set of 5 overprinted stamps was released to publicize the balloon flight competition from Brussels to Heysel on September 13, 1953. A total of 2,500 sets were so overprinted.