The new zombies maps are quite strange. I don't think the timeline is actually supposed to be relevant. Treyarch, for the most part, is really good and filling in the holes and making sure that nothing is off, but they may have screwed up this time. There's a piece of paper that says 1975, there's the Berlin wall, there's that building that finished construction in 1969 that is under construction on this map, the M16 and AK74u are on this map, but the MP40 is also here... The timeline simply does not make sense, but maybe that's the point. They traveled through time, so they could have screwed things up.