This is a medical advertising postmarked mailed in 1961 to a doctor in West Virginia, USA.
On the back of the card these words are printed:
Here in Portugal, our special stamp commemorates the life, 5 centuries ago, of Prince Henry the Navigator. Today PENTOTHAL too has charted new worlds of progress. You'll like its dependable, easily controlled intravenous anesthesia. Use it soon.
(Here is the stamp on the postcard.)