Wiki says 1949.
And H&K weapons in Fallout include the fictional http://images1.wikia.nocookie.net/__cb20110211035547/fallout/images/7/77/Fo1_10mm_SMG.png and the http://images3.wikia.nocookie.net/__cb20110308165930/fallout/images/3/3b/Fo1_Gatling_Laser.png, aswell as the real-world http://images2.wikia.nocookie.net/__cb20110308054327/fallout/images/3/38/Fo2_H%26K_P90c.png, http://images4.wikia.nocookie.net/__cb20050511052113/fallout/images/2/29/Hkcaws.gifand http://images4.wikia.nocookie.net/__cb20050511052831/fallout/images/9/9c/HKG11E.gif from the older games. So basically, in Fallout 1 and 2, they used real-world companies with either fictional weapons or more modern prototype weapons that got mass-produced in the Fallout universe (like Pancor Jackhammer)
Real world weapon companies known in Fallout 1 and 2:
http://fallout.wikia.com/wiki/Heckler_%26_Koch
http://fallout.wikia.com/wiki/Colt
http://fallout.wikia.com/wiki/Glock
http://fallout.wikia.com/wiki/SIG-Sauer
http://fallout.wikia.com/wiki/Winchester
http://fallout.wikia.com/wiki/Rheinmetall_AG
http://fallout.wikia.com/wiki/Rockwell
Notice how most of their weapons are fictional.
And I am all for "old" weapon designs, with wood and all, because I think such weapons are much more beautiful. I liked the service rifle in F:NV - semi-auto M16 with wooden parts.
Don't forget these:
http://fallout.wikia.com/wiki/FN_Herstal (which oddly they have in Fallout 2 but state that the P90 is HK, then Fallout Tactics gets it right)
http://fallout.wikia.com/wiki/.44_magnum_revolver_(Fallout_2) (not sure if S&W is a canon pre-war company but it's mentioned a lot on the wiki on various pages)
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All we know is that the divergence happened sometime between 1945 and 1969 (1961 depending on source). So technically any design up until that end date could be considered fine.
There are some weapons that don't state the manufacturer in the games such as the http://fallout.wikia.com/wiki/Desert_Eagle_.44_(Fallout) (IMI) but are clearly later designs and non-prototype.
The http://fallout.wikia.com/wiki/M60 (USORD) could be cutting it close to the divergence. More importantly if this is canon, there is no reason AK-47's and M-16's shouldn't be. Some people suggested that AK's shouldn't be in the games, well guess what the 47 part stands for?
Then there is the http://fallout.wikia.com/wiki/Light_support_weapon (Enfield) which is based on a real world service weapon, the L86A1 LSW. I will exclude the http://fallout.wikia.com/wiki/XL70E3_(Fallout_2) which actually is the prototype of the L85A1/A2 which is a variant of this same weapon family known as SA80 (Small Arms 1980's).
Also consider that 7.62, .223, and .44 Mag (these are the ones just from the first two games) were developed after 1950, and that some of these companies didn't exist prior to that either, Glock for example.
So my point is, where do you draw the line? Who's to say that some things wouldn't have turned out the same if history had veered off into an alternate universe? Besides if you look at all the other games in the series, some modern firearm designs are in them. Having that decade or so of unknown leeway when the divergence happened helps, and they shouldn't ever change that. I'll concede that it's better that when they use a modern gun design they don't use it's proper name.
On the other hand, keep in mind that firearms if well looked after can last generations, and just because a design is older doesn't mean that it hasn't been copied and brought into production again by companies such as the Gun Runners.
For me at least this topic is a moot point that I can overlook. Food for thought at any rate.