As I noted, I could not see the details on the frame because the pics (the pics are too dark when viewed on my smart phone). I just borrowed and fired up a laptop at home to look at them again and you are right about the missing "GTP" pantograph on the seatpost clamp ears and the suspiciously dull seatstay finish which should be a brilliant chrome on Montellos.
Also missing is the light color (usually white on their blue bikes) infill on the GTP panotgraphs and the flowers cutout on the frame.
Now seeing all this, I agree that this looks suspiciously like a Treviso, maybe made to look like a Montello by stripping the chainstay paint to reveal the dull chrome under the paint (I think the Pinarello frames were chromed all over, but the painted over areas did not have the good finished chrome on them.). The Treviso is still and excellent Pinarello, made with Columbus SL tubing, instead of the SLX used on the Montello., but market value is up to 1/3 less than Montellos. This possibly faked one to look like a Montello would even pull in less because of the "tampering" done to it. Do not buy this one, unless the seller sells it for a bargain. It's still a good bike and maybe it can be restored to be a Treviso again with just a little bit of matching paint on the chainstays and a Columbus SL decal on the seat tube. Heck, the downtube decals still looks good, and that's a plus on any Pinarello.