Good eye, Khj! What an interesting stamp.
The character that Khj mentions is this one I've highlighted:

which is the Chinese character for "5", so that's one of the reasons this stamp doesn't really make sense. The design does appear to be lifted from the 5-mon stamp (Scott #1), with a change in color and in the numeral.
I have this older, kind of creaky catalog produced by the Korea Stamp Society (2nd ed., 1990), in which it refers to a "2 mon green fantasy" in the notes of the 5-mon issue, so perhaps that's your stamp.
I agree with Khj that it might be worth trying to contact the KSS. I should add reluctantly that I've tried to contact them several times by email with questions about membership info, etc., and have never received a reply.
Do let us know what you learn, if anything.
I wouldn't toss it out. It may be something that a specialist of forgeries would be interested in.