I think you are asking about Perfins or PERForated INitialS (or INscriptionS) or in German Firmenlochung (firms (companies) punching).
They were used by larger companies to deter postage thievery by employees.
I can't see the whole stamp, I have a slow connection and sometimes (most times) larger file-size images from Imageshack or other image hosting sites time out on me and stop loading. I need a better computer and connection. I can see some holes punched in the top area of the stamp though.
If on cover they are usually valuable. Off cover, for Sweden, I do not know.
Some people collect them and find them fascinating. Perfins were kind of dismissed by the general collecting community in Canada years ago when it came to light that some were fakes. Or thought to be fakes. It took years for the truth to come out that most of the fakes were not fakes at all but different versions (25 different OHMS dies) of the same perfin produced by different machines.
See Ken Pugh's publication on the OHMS and G official perfins of Canada:
:
http://www.kenpugh.ca/PhilatelicSer...elease5.aspxI am not sure what they would be called in Swedish on Tradera. Perhaps they would use the Perfin word also or some would.
In the Scott's and Unitrade catalogues they are found in the back of the book under the 'O' numbering system (for official). Private companies perfins are not in the general cataloges and you will need specialized catalogues for them most likely.
Catalogue:
Swedish Perfins (Bergman, Lagerwall, Swenson, 1981; Svenska Skyddsperforeringar) The primary catalog for this field.
There are some web sites to start with:
http://www.angelfire.com/pr/perfinsoc/http://www.norbyhus.dk/perfpage.htmlScandinavian Collectors Club
http://www.scc-online.org/libsweden_b.html