Of course the bomb doesn't look very authentic because the nose is highly un-aerodynamic. You always want the center of pressure to be near the tail for stable flight.
The Dr. Strangelove bombs had big tail fins. That would serve to move the COP aft, but the unaerodynamic nose would create instability anyway from turbulent boundary flow, so yeah.
I don't know if pictures of real US bombs were available in 1963...
Likely not. Nowadays we know that some of the thermonuclear weapons carried on the B-52 had casings with relatively blunt noses, but not any with exposed longerons and shapers. I think Ken Adam simply went for something that would look "different" from a conventional bomb in such a way as to emphasize the menace. He had served in the RAF and I'm sure he was quite familiar with what bombs look like in general, and why. But when you look at his Bond set designs, you realize he wasn't always after realism.
There is, of course, the famous story about the USAF objection over the highly realistic depiction of the B-52 flight deck. Initially there were accusations of espionage, etc. It wasn't until Adam produced his research, including pictures from a popular aviation periodical and other open sources, that the Air Force realized they hadn't done as good a job as they'd thought of keeping the B-52 a secret. However, I don't think that applies much to the design of the bombs.