Old news. HTC has said before the only reason it's not there is because they don't have any info on what the interior looks like. They left it off for this reason. Some folks a while back supplied some info, but the A20's not going to be redone for a while yet. Maybe when it's upgraded it will get the extra gun.
I don't know about the Boston III, I don't think it had one, but the Havoc had a ventral gun. The thing is that a lot of crews ditched it in order to save weight (going from memory on the previous threads about this)