This item is now known to be bogus: identical overprints, clearly using the same handstamp, have been found on Irish stamps and "do it yourself" frames.
It is therefore no longer included on the web site, though this page is kept as a "ghost" for reference.
"Tidal Testing" is where researchers wish to see where tidal currents go out on the open sea (especially from the mouths of rivers). A common means of doing this is to throw floating devices into the water with a message attached asking the finder to return the whole item back to a specified research address. It appears that sometimes the postage is covered by the researcher.
One GB stamp is known with an overprint giving the impression that the return postage was covered by inserting in the device (presumably in some waterproof manner) a 1/- postage stamp, with an overprint to stop it simply being kept by the finder of the device. J Evans illustrates a 1/- George VI stamp overprinted "TIDAL / TESTING / SEA BORNE / BOTTLE". However this is now known to be bogus.
(illustration courtesy of Jon Evans)
However, since under GPO rules any unauthorised overprints render a stamp invalid for postage, the GPO must have been aware of, and approved, this usage as valid for the return postage, in which case this overprinted stamp has a reputable philatelic pedigree and is not a "phantom".
source: http://www.useless.connectfree.co.uk/help/help.htm (site no longer available) 23 March 2002
page last updated on: 23 June 2006 , 21 December 2010
gbos: GB Overprints Society