The front of the envelope is the usual from the British Royals.
The back of the envelope feature's Prince Philip's badge.
Inside is a letter from Prince Philip's Correspondence Secretary, Suzy Lethbridge. Prince Philip's mail used to be handle by his Assistant Equerry. But when he retired, Prince Philip reduced his staff and his Assistant Equerry went to work for Prince Andrew.
The letter is similar to Prince Philip's past birthday replies. He does change up the message on the letter from year to year, but the messages often get repeated. This year's message is the same as his 2016 reply.
The letter says:
Dear Miss Daly,
Prince Philip has asked me to say how much he appreciated your good wishes for his birthday.Your Sincerely,Suzy Lethbridge
Correspondence Secretary to H.R.H. The Duke of Edinburgh
Prince Philip's office sends out the same letter to everyone who writes for an occasion. This is why only the month and year is given for the date and there is nothing printed on the letter personalizing this to me. However, this is the first birthday reply I have gotten from Prince Philip that has "Dear Miss Daly" written in. It's only a little thing, but it is nice to have that personalization.
I do wonder why the change now, after years of not personalizing the letter? I know from experience that the more overwhelmed an office is by mail the less personalized they tend to be. So, could it be they have more time on their hands? Less mail than past years?