Early units had the DSI logo. As of a couple years ago they changed to using the Sequential logo on all synths including the Rev2 and OB-6. (Exception being the OB-X8 of course, which has the Oberheim logo)
As for the Oberheim logo on the OB-6, they seem to always have had the Tom signature silkscreened on the OB-6 but a year or two ago some shipped with an Oberheim logo decal applied over the signature. I’ve never heard of one with no logo OR signature but I’ve seen stock photos without it, perhaps it was edited out or it was an early prototype or something? I don’t know that any shipped without the signature at all. I don’t know why some shipped with an Oberheim decal added but then they seem to have stopped doing that, either.