Could be the Rh130, since we're seeing more and more signs the M256 will be superseded by a different gun (XM360 or other; 120mm or larger) in future MBT types. I don't think it's the Rheinmetall L52 howitzer gun that was recently seen at AUSA2023, fitted to a BAE M109A7, because Yuma Proving Ground would have been the choice contractor for such field testing.
@Manic Moran might disagree with me, though, since he's better acquainted with these entities and the acquisition/testing processes, and I'm just making quick assumptions.