The only rifles so equipped (that I can recall) were imported in the 1980s by Navy Arms. Try giving them a call, they may have done that modification in-house. There was a picture of the modified bolt *carrier* in an old American Rifleman article, can't recall the issue (pre-1990, I believe). I'd like to know how it was done, too, if you find any more information please post it here and on "Survivor's SKS Message Board".