What you're looking at
The ACS550-U1-125A-4+B055: The '+B055' suffix is an ABB factory option code applied to the base ACS550-U1-125A-4 catalog line; it travels with the drive as ordered and is part of how the MPN is specified into the BOM, not a separate component.
Same-frame cross-shop: ACS550-U1-072A-4+B055+L512
ACS550-U1-072A-4+B055+L512 matches this drive at 34.02 kg; ACS550-U1-059A-4+B055 weighs 36.29 kg and ACS550-U1-096A-4 weighs 19.96 kg, so only the 072A shares the chassis class for direct cross-shop.