indeed the alloy thing holds the rear main neoprene seal in it.
if you look at my 2T you can kinda see it almost in line with the orange round thing holding the engine on :
if you've gotta replace the rear main seal, out comes the trans dude....if you've got the sump off you'll have had to have the two gearbox stays either side off, so, a few more bolts and a tailshaft are not going to hurt...
PS : make sure you put some sealer when you put the sump on - i put it all around for good measure BUT the OEM toyota books say put plenty of sealer where the alloy plate finishes and the block starts...