I have two U.S. Mark of the Wolves MVS carts, one in solid green plastic case, the other in translucent green plastic case. The solid green cased one has the original serial number printed on the label, the translucent green one has had the serial number cut out. I don't know about Japanese versions, but I would assume that they have serial numbers just like the U.S. carts.
What you need to do is to open the cart and check the two PCB boards inside. If you see EPROMs on one or both boards, it may be trouble. I checked both carts that I had, and neither one has EPROMs or wires on it. All the PCBs boards themselves have SNK info printed on them, and all the chips (except the two Program chips in both carts) are commercially made and labelled.
BTW, RyoGeo first broke the news about bootleg Garou carts here in the forums. He also set up a cool web page all about bootleg carts, here:
http://members.aol.com/ryogeo/Garou_Boot.html
Thanks to RyoGeo for the great info!! And neoluva2001, let us know what you find out . . . hope its a legit cart!!