Also what is a "blanking plate"
Upon deselection of reheat, the priming, dump and shut-off valve (PDSOV) is closed, isolating the fuel system from the manifolds, but residual fuel in the manifolds is dumped over board on shut-down.
The blanking plate was a square piece of metal inserted between the joint of the reheat purge line.
This effectively blocked any fuel from being dumped overboard on deselection of reheat, thus stopping the plume of tell tale vapour.
This left residual fuel in the pipework and on a rapid reselection of reheat the residual fuel would ignite and cause a reheat surge.
As flipflop stated, this also caused the manifolds to start bursting due to the pressure on the system.
To stop this happening, some clever sort came up with the modification to drill a 3.2mm hole in the centre of the blanking plate. This allowed the fuel to trickle into the purge line, not so much to cause the plume, but enough to stop the surge.