for everything WW2 and before it is
the fallout universe is our universe untill some time after ww2 it splits, it is never specifically stated at what date our timelines split, but our world and their world are not parallel universes untill sometime after ww2
in other words every single thing that happened or was developed is exactly the same with the same results in their world as ours
there a couple of noted exceptions of pre-war differences
sunset sasparilla, that was 1918, not in our world
vikki & vance, 1932
well had to make a soft drink up cant use any real world ones, licensing etc
vikki & vance obvious reference to bonnie and clyde, just was much better to have a reference rather than having the real
these are just gaming things.
as far as not having real world weapons goes, personally i dont see a reason why the guns in their world would not like similar to ours
weapons are not designed with art or culture in mind (except some civvie ones) military weapons are designed for their functionability, they would have the same requirements as we would. They dont have to be called the same name but they should look similar to ours. Such as the r90 in FO3 and say the service rifle in NV. yep they could look more different to real world counterparts than ours, but they should retain the same basic shape and functionability characteristics as in our world.