This is interesting. But this doesn't really surprise me at all. There seems to be 2 versions of the BGVP DMGs. Mine has no vent on the back end of my DMGs. But
These do. Here we have a revision that went unnoticed. These I suspect are the same as NiceHCKs M6. So some of you guys got the new DMGs and some don't. I suspect early adopters of the original DMG got the ones without the vent in the back of the housing. Newer ones will have this little minor detail including the M6.
BGVP has done this before. They released a cheaper hybrid DS1 with a mid forward tuning only to realize it might not be so popular among the international crowd so they retuned it and brought out a 2nd version. Don't know if it is BGVP or the OEM that makes them that is guilty. In any case. The one you want is the one with the vent on the back of the DMGs.
To be on the safe side. Get the NiceHCK version as all of their M6 will be this newer version.