Lever Group Portafilter O-ring
Many other manufacturers used the Gino Rossi lever group on their "Leva" models to avoid having to research, design and manufacture a similar group of their own. Check for corresponding parts from the diagrams provided with your existing machine to be certain this is the correct o-ring.
This version gasket is in fact quite antiquated, and all manufactures now recommend the flat version as it provides a larger surface area for sealing.
Other manufacturers that have used the group that use this o-ring include: Bezzera, Brasilia, Faema, La Pavoni, Nuova Simonelli, the PRE-2012* versions of Victoria Arduino Athena (distributed by Nuova Simonelli, as opposed to the original antique Arduino) and Wega.
*Nuova Simonelli revised the Athena group head for models manufactured after 01/17/2012 to use a square gasket, not currently available through Espresso Parts.