@Little_Boot:
Last time I checked, there isn’t a technology for that one.
Ah. I didn’t realize that you only wanted technological upgrades from the rulebook.
The suggestion I’d like to make is an overall approach you could consider for all countries, rather than a specific tech for Germany. The suggestion would be for you to regard your special house rule as being for an alternate-history scenario – a timeline in which technology progressed a little more rapidly than it did in actual history. That way, you can give each country whatever tech upgrades you want at the start of the game without having to worry about whether it’s historically accurate to do so.
I think this would solve the problem created by trying to give countries realistic tech upgrades at an earlier point of the game than would have been possible historically – your reason being that “it might be cool and historical” to do so. The trouble is that premature tech upgrades are certainly cool, but they’re not historical. Tech upgrades are, by their very nature, later improvements over less capable earlier technologies (the starting technologies at the beginning of the game). Moving the techs from the later phases of the game to the beginning would make them the starting technologies. This means that you wouldn’t be able to upgrade from them (since you’re already using the upgrade), but more importantly it isn’t historically accurate. To use as an example the Type XXI Elektroboote which I mentioned: Germany didn’t use these super subs (if you want to call them that) at the beginning of the war for the simple reason that, in the real historical timeline, they didn’t exist yet. The same problem applies to other techs too.
So the way to give tech upgrades to countries ahead of their time is simply to go for “cool” without trying to be “historical”. Which is perfectly fine – alternate history scenarios are a lot of fun, and house rules a good way to explore them.