This is a takahashi side-by-side mounting plate. I believe that it came from a EM10 mount though I 'think' is compatable with most all Takahashi mounts. It is designed to have a main scope mounted to the central part of the plate, and then a guide scope mounted to the side part of the plate.
Losmandy lists his side-by-side at $240. This one is not as sophisticated - yet maybe a better choice if you've got a compatable Takahashi mount.
See image for specifics.
Pick this up in Oakland for $95.