Berlin Clearing Office Cachet?? Help Needed

I've spent the last few weeks trying to decipher the smudgy round cachet on the front of this 1921 registered cover from Narva, Estonia to Heidenheim in Germany. All I can work out is "Berlin ...something" at the bottom and "Post....wad..." at the top, with the Eagle in the middle. It looks to be the same size and general layout as the "Zollamt." cachets from German customs clearing offices, but that's as far as I've got.

The detail may not be too helpful but it's enhanced and sharpened as best as I can.

Anyone got any good ideas?

Postüberwachungsstelle = Postal Inspection Office

I'll see if I can find that specific cachet in my references when I get home from work today.

Thanks Postmaster!