A large number of these photos were assembled from the RG-77-BT collection in the Still Photo collection of the National Archives II building in College Park, MD.

““Thin Man” Plutonium Gun Type Bomb Casings in 1944. In the background you can see “Fat Man” casings. It is unknown whether they are the early Model 1222 “Fat Man” casings, which required 1,200 bolts to assemble, or the later Model 1561 casings which were substantially easier to assemble and which were used for the production versions.”

Many more interesting old photos to be found here