Some time ago the Skeptical Inquirer presented some information regarding the assertion by a guy that used to work signage and such. He thought that the Alien Autopsy Danger Sign looked a little to new for the supposed age of the footage. His guess was that the sign came from around 1967.
These signs came from photos of a military missiles. The first one was called Mike or The Sausage. There are identifiable people in this photo which is very helpful. This bomb was set off on November, 11, 1952. This photo had to be taken before that.

Next we have a bomb called Castle Bavo Bomb which was set off in March, 1 1954. This sign was located in the storage room which housed the bomb.