Tripped across this model at the San Diego Air & Space Museum's (SDASM) Flickr photostream. It is a photo of a model, featuring the caption "Saturn Model - Saturn D." The little I have been able to find on the "Saturn D" is from the NASA Authorization for Fiscal Year 1967 Sentate hearing notes, in the context of Lunar Mission studies, Page 555:
2. Saturn D (Nuclear Rocket Upper State) Mission Studies," Lockheed Missiles & Space Co., August 1962, contract NAS8-1601
3. Saturn D Nuclear Rocket Upper State Mission Study," General Dynamics Astronautics, August 1962, contract NAS8-1600
Here's a link and the photo:
https://www.flickr.com/photos/sdasmarchives/31236821642/
Came across additional photos (larger versions are available at the addresses linked above each photo):
https://www.flickr.com/photos/sdasmarchives/31236822422/
https://www.flickr.com/photos/sdasmarchives/31236824022/