Here is the photo in question. I am examining the Mercury camera with Red Williams, the camera builder. In the second photo, taken from the news after I left the cape, Red is shown with two of the astronauts.
In the background of the first photo, you can see the second and third capsules undergoing checkout. After this photo was taken, I went into the white room with one of the photo crew members and had a chance to stand up into the capsule. The door was facing down, so you just crawled under and stood up and you were inside the capsule.