It is unknown. The HP 9mm was introduced by Browning in the US in 1955, but they only started to keep production records beginning with 70,000 in 1957.
Your gun was probably made by FN and sold outside the US prior to 1955. Early 50's would be a good guess.
To verify my information, you should notice that while the gun is stamped Brownings Patent, nowhere on the gun will you find the words "Browning Arms Co." which began in 1955