The 727 has a 2-part nose gear door. The rear part by the strut stays open when the gear is down. The front part only opens when the gear is moving. In the picture, the front part of the nose gear door is closed. On a 737, the nose gear door is one piece, therefore the whole assembly stays open when the nose gear is down. Therefore, the plane on the right is a 727.
Hope that helps.
By the way, the plane on the left is an L-1011.
Sorry, I had to do it.