The front of the envelope is the usual from the British royals, featuring the postmark from the court post office at Buckingham Palace.
The back of the envelope features the black crown/Kensington Royal that is currently used by the Cambridges.
Inside is Clarence House's typical birthday reply, a photo card with a typed message on the back.
The front of the card features a photo of Prince William by Norman-Jean Roy. The photo is from William's 2017 interview with British GQ. You can read the interview here.
This photo was also used on Prince William's 2017 and 2018 birthday replies. It is quite typical for Clarence House to reuse photos for multiple events, especially the adult's birthdays.
The back of the card features the black/crown Kensington Palace at the top, along with a typed message. The message is slightly different than last years.
The message reads:
Back to Replies - Main PageThe Duke of Cambridge is most grateful to you for writing as you did on the occasion of his 37th birthday.It really was most thoughtful of you and His Royal Highness sends you his warmest thanks and best wishes.