Assuming the Elinchrom Rotalux AB rings haven't been redesigned since I got mine, be warned that they do not mate with the Einstein's mounting fingers well at all.
While most AB speed ring inserts have a dished profile, the Elinchrom AB ring has a stepped profile. The vertical side of the step prevents the light's mounting fingers from opening fully:

This is as far as the accessory locking lever on the Einstein can travel:

The mounting fingers are not locked with the lever half way up the slot.
This is a *very* surprising design blunder on the part of Elinchrom.