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My experience with dealers at shows has been that they will stiffen their jaws and not accept any contrary information...even when shown in a catalog.
Not necessarily true. If you are unknown to them, i.e., random walk-up customer, then probably. However, if based upon past communication and/or purchases, they recognize you as a specialist that knows your area, most are willing to at least listen.
I have helped more than a few dealers with identifications and cancel attributions, and most are grateful for the assistance, even when you're telling them the lotto ticket they thought they had is bogus or misidentified.
Also, it depends on how you broach the issue. "Hey, this stamp is fake!" or "You've got this identified wrong!" is unlikely to endear yourself to a dealer. I usually ask "Do you mind if I give you some feedback on this item?" and then I proceed to clearly explain WHY I have the opinion that I do, ideally accompanied by showing them images from my own collection so they can compare and contrast the item in question.