Same idea as a Perfin, has a similar strategy.
We list them in a differing hiearchy, we call them "Commercial Overprints"
Hope that helps.
Actually the delineation gets dodgy here,
"commercial overprints" generally refer to fiscal use.
You can read the chicanery for authorised and unauthorised overprints here
http://www.gbos.org.uk/index.php/Country_List/16