Ivor Parrington has sent the Seaham Family History Group a 1976 photograph of the now demolished acoustic mirror at Seaham.
This is the first picture that I have seen of it. As had been reported, the design looks like the World War I sound mirrors at Boulby, Redcar and Sunderland in the northeast, and the one at Selsey in Sussex. The sketch of the reported mirror at Hartlepool looks the same too.
This would date the Seaham mirror to about 1916.
Apparently there is nothing left now, but the location has been identified as being here:
(thanks to Raymond Thompson of SFHG)