I agree with Tom Dockery's opinion that this is a Sekai, most likely a 5000. I have a 4000 from around the same era (late '70s) with a very similar serial number: M6K20108 (same format, anyway). The lug detailing is also similar, except Mike's has a couple of extra nice touches. Mine is also completely chromed so that certain details can be revealed as chromed when masked off during painting.
Sekai DT decals are available at Yellow Jersey, as A. Muzi was also an importer (I think he had something to do with spec'ng components for some of the models). He doesn't have a "4000" or "5000" decal availabe, but has others with which one could cut & paste or use as patterns. Andy told me that many buyers preferred to strip off the decals. IMO the decals are not nearly as elegant as the frame, so I can understand.
I think Don Gillies has collected some Sekai wisdom as well.
Dave Ross
Portola Valley, CA USA