You lucky thing! Armagnac is the oldest brandy in France, with a history going back 700 years.
Armagnac is predominantly single distilled as opposed to Cognac which is double distilled. There are other differences such as the grape varieties used; 10 different grape varieties are permitted in the Armagnac AOC appellation though most producers use 4 principle ones. In Cognac they tend to use just one variety; the Ugni Blanc grape.
Armagnac comes from the beautiful rural South West region of France covering the departments of the Gers, the Landes and the Lot et Garonne.
Production is small, traditional and artisanal and each Armagnac is different. The 1951 Armagnac that you have is the result of the 1951 harvest alone and should be drunk as a digestif after a meal to be best appreciated. Turn it in the glass, breathe in the aromas (prunes, vanilla, crystallised fruits) then sip it gently and enjoy!
I don't personally know this producer, but suggest that if you want to know more, you could look at the armagnac.fr website.