self center flange what the différence betwen mod:E162500and E162600 thanks

The Quickcenter X1 has a short step on the back plate to assist in centering the buff prior to installing the top plate. This step does reduce the amount of threads in the top plate in order to prevent contact with the back plate when installed. If the X1 will be used on a tool with a short arbor shaft it may have less than ideal thread contact which could result in stripping the top plate when tightening. In this instance it would be better to select the Original Quickcenter design which has a flat backplate and a taller centering step on the threaded top plate allowing for additional thread contact.

In conclusion:
The Quickcenter X1 is designed for faster wheel changes on tools with long arbor shafts.
The Original Quickcenter is a universal fit and works well with both long and short arbor shafts. Wheel change times might be slightly increased.