Couple things to check on this frame: what's the seat post size (I believe Matrix AKA Cromor will take a 27.2 but if it's smaller then that decal is suspect, too). Then pull the fork and see if there's "rifling" in the inner butt of the steerer (you don't have to pull the fork since you can check that with just a good flashlight) but to see if the steerer is stamped with (hopefully) the same "M50" as the BB shell.
AFAIK a Matrix fork might NOT have gotten the "rifled with spiraled ridges" steerer used on most every other Columbus frame (SL and up) but there will always be exceptions. Are the fork-ends stamped GPM too (or Campy)?
The fact that is has some chrome on the frame (DO surfaces and chainstay) might make a full-chrome fork more likely...maybe.
Last if there are some more pix of Kotter frames from the same era to compare with you might find matching (or not matching) frame details, like how the cable routing is done (with guide tube) in the top tube and the nicely done transitions on the DOs-to-chainstays joints.
It MIGHT be a lower-cost frame (Kotter or not) but looks to be nicely built, so it wasn't a slap-dash product. I agree it's very unlikely this is a Pinarello of any type.