The photos were originally used for a piece the magazine was doing on the prime-time animated series. They only used three photos. However, the photographer, Allan Grant, took 850 photographs for the assignment, where he documented every part of the goings-on in the studio.
Check out a few of the these rare gems:

Story meeting. Standing, left to right: Joe Barbera, Bill Hanna, Warren Foster, Seated, L. to R: Dan Gordon, Alan Dinehart, Unidentified, Michael Maltese, Alex Lovy.

Mel Blanc (Left) and Allan Reed recording the voices of Barney Rubble and Fred Flintstone, respectively
“Hey, Barn! Life has 15 Unpublished Photos of Us!” http://t.co/tSR11G6DJT
RT @GeeksandBeats: “Hey, Barn! Life has 15 Unpublished Photos of Us!” http://t.co/tSR11G6DJT
#ThrowbackThursday fun: Unpublished photos from #TheFlintstones! http://t.co/gKy5hx2xLg
RT @alancross: #ThrowbackThursday fun: Unpublished photos from #TheFlintstones! http://t.co/gKy5hx2xLg
RT @alancross: #ThrowbackThursday fun: Unpublished photos from #TheFlintstones! http://t.co/gKy5hx2xLg
RT @alancross: #ThrowbackThursday fun: Unpublished photos from #TheFlintstones! http://t.co/gKy5hx2xLg