Not necessarily, the Dell Model Ms didn't say IBM anywhere on them - they instead said Lexmark, IIRC.
Also, IIRC, the earliest Model Ms didn't say IBM on the label, and had a drastically different label layout.
But, something that the OP didn't mention, so I'll go ahead and ask it... is the keyboard clicky, or rubber dome?
If it's rubber dome, I'd be inclined to believe that this isn't a Model M at all, but rather, a relative of the KeyTronic KT2000. (The KT2000 has Windows keys, but KeyTronic's been producing that basic design since... well, not long after the Model M came out.)