Catherine Langford mentions that the 1945 gate experiments were being conducted as a search for a weapon, which indicates that they occurred prior to the use of the atomic bombs against Japan (August 6 & 9, 1945), and possibly prior to the end of the European war (V-E Day, May 8, 1945). But Ernest Littlefield's journal notes make mention of the United Nations, which didn't exist until October of 1945. As Littlefield would have been on another planet by that time, It is unlikely that he would have known about the UN.
After the rocks have fallen down in front of the Stargate, Jack touches one of them, revealing that it's fake (as a feather).
Dr. Jackson mentions that he can tell that the holograms of the elements are particular elements, because the number of electrons determines the element. The number of protons determines the element. The number of electrons can change if the atoms of the material are ionised (gain or lose electrons). However, in un-ionised atoms, the number of electrons is the same as the number of protons. To be fair, Jackson is not a physicist or a chemist.