Near impossible? Where do you get that from?
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The content of the letter you posted has nothing to do with risidue of a print being left behind when the print is lifted.
The FBI found not even a trace of a print on 11/23/63. When the rifle is returned to the DPD, Day has prints of Oswald's hand and palm available.
How easy would it be to stick such a print to the rifle and thus obtain the irregularities onto the lift?