Originally Posted by
dedhed
Many of the Shimano cartridges have no real "lip" on the NDS cup so a spacer cannot be used. The NDS cup only has to go as far as needed to center & support the NDS of the cartridge.
Like this UN 54 on my hybrid and most UN 26.
If you look at the photo of the OP's original UN 72 BB the NDS cup has no lip.
BB spacers barely fit over the threads, so that barely there lip is all that is needed, admittedly, not all BB cups have a lip but as LV2TNDM points out, with this style of bb the left side spacer can be a redundancy, the cup really serves a wedge function and can do that sans spacer, though from an aesthetic stand point I like the spacer there if the cup has a lip like your BB does, just makes it look more like someone knew what they were doing.