If you import a part from McMaster, then right click on the component in the tree, go to the properties window, and change the "Part Name" the "Part Number" field will also be changed to the same once you close the window. I would think the standard action would be to have the Part Name appear in the Browser tree, but retain the McMaster part number under the "Part Number" field inside the part properties. Currently you have to do some copy/pasting and reopening of the properties window to retain the McMaster number.