The origin of this overprint is currently unknown.
From an archive search on the Internet I found a firm called George Cohen & Sons who are described as Shipbuilders, Hydraulic Engineers, Scrap Iron & Metal Merchants.
A Post Office Commercial Directory of 1865 lists no fewer than 88 firms using the name Cohen. Without more information it is unlikely that the true user of this overprint will be established.
1872. Unofficial overprint type 11 in black.
There are a number of later perfins that indicate possible identities, such as:
See the Philatelic Links and References page for more information on Perfins.