I hit this in the other post where you brought it up, so I will hit it here as well. They were used. Regardless of how much, they were. If use is a metric for determining credibility in AH, the B-239 would have never been added.
I asked if there was a shot at the K6 with the gun wing, but there is just too much ahead of that in the works. So, this is the middle ground for me, personally.
On a sort of related topic, are the 20mm rounds fired by the MG151 the Minengeschosspatrone 151 ohne L'Spur type?