It took a while to find this. But I remember seeing it when it was first uploaded a while ago. If anyone wants to check their Hulkbuster to confirm that the screw is in the same place, then this is probably how the jackhammer arm is intended to be installed.
https://www.youtube.com/watch?v=qKDi4AKc2JM
looks like HT designed it with a swappable arm in tbe first place. the jackhammer should have come with the first release