If it's an Evo X the part is 3200A141 and it's just called the "COVER,T/F CASE,UPR". It doesn't look like there's a gasket involved so if it's leaking from there it should just be a case of draining the box, removing the plate, cleaning it up and then refitting it with some gasket gunk.
Are you sure that's the bit that's actually leaking and it's not just running down there as it's more likely that it's leaking out of the driveshaft oil seal?
I had a leak from that plate on mine a couple of years ago. Some debris knocked it and broke the seal. Original plate put back on with fresh seal and all has been well since. Though as mentioned above just make sure that is the area of concern for your car.
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