The version in question is the Bf-109G-6/U4
It had a retractable tail wheel, and so would not work as just a gun package on the AH 109G-6.
Yes, it is U4.
Squire, are you serious here?

First of all, G-6 is the most produced variant of the 109-series. As HTC isn't keen on adding multiple G-6s, you would still say with a straight face that a difference in tail wheel is a reason not to have it, riiight.

Second, you've read some bad source, all G-6s had fixed tail wheels. And third AH's G-6's tail wheel retracts in the current version.

Yes, it is a bug.

As for adding it, well, im not sure what it adds to the game considering we already have the 109G-14. Thats not to say it could not be looked at, but appreciate that there are many varients of other a/c as well not in Aces High, and there is no way we will ever see them all. HTC makes decisions (it seems to me I dont work for them), on varients that fill gaps.
You talk like it would be something new and difficult to add. It was removed, I'm sure it wouldn't be too hard to put back in the game. What it adds to the game, well take a look on my previous posts and to some others have also stated that they prefer it over the 20mm. The gun was there in enough numbers to warrant inclusion. Whether it was there in numbers or not in 1943 shouldn't really matter. I'm sure the La-7s with the 3xB-20 setup weren't the first La-7s to roll off the production line either.