It's the FASA games that assume that there are several different species inside the Klingon Empire who enjoy the right to call themselves Klingons. The idea also appears in some novels, most notably in the two John M. Ford ones because Ford was involved in writing FASA background material, too.
The FASA species are pretty close to the ENT excuse, actually: "Imperial" Klingons have the full ridges, but people with mixed blood are still Klingons if they have at least a bit of Imperial blood in them. They just lose their ridges if they are tainted with human blood, as the result of interspecies mating or deliberate devious manipulation. And ENT emphasizes the latter means of giving human DNA to Imperial Klingons...
Timo Saloniemi