The reason you cannot find differences is because there is really not any.
The only differences are Packaging options from different makers/suppliers.
Clear center only
Metal center only
Both centers included
Standard Drip Tip only
Screw in Drip tip only
Screw in drip tip w/standard Drip tip adapter
Any given combination may be offered supplier dependent.
Kayfun Standard Volume is 4.5Ml
3.1ES
Lite
Plus
V2
All = 4.5Ml as a standard