The Z58 is much more compact - just a little bit longer than a stanard LOTC.
The Z48 uses a different click mechanism designed for the Military WeaponLight user. The TailCap is easier to rotate while gloved or wet etc. It is shrouded to help prevent accidental activation and ensures more deliberate clicking. It is longer and thicker.
This photo gives you an idea of how the Z58 relates the standard LOTC.
It appears to me that the Z58 is the TailCap you are after.