Those tabs won't work on an Omega D negative stage, but it looks like an adapter for some kind of 4x5" carrier for 6x8cm. I thought it might fit in the Omega Distortion Correction Attachment (tilting neg stage), but it doesn't have the holes in the right places.
On the other hand, it could be a mask for another Omega D carrier that doesn't have to go all the way in. Like maybe if you have the rectangular 6x9 carrier, this one could fit between the two halves with the tabs sticking out the front of the carrier.