Of course they knew the prints weren't Oswald's. But the conspirators had to go thru the motions for the cameras and onlookers. They decided which print(s) to use after they got Oswald's corpse to put fresh prints on the rifle, then they took his prints as reference to match the new prints they lifted from the rifle stock. Then they replaced any 3 x 5 index cards with the 1 post-mortem partial print. I guess corpses don't "print up" very well.
Then they replaced any 3 x 5 index cards with the 1 post-mortem partial print. That didn't happen....Hoover was in the driver's seat, and if he wanted his finger print expert to swear that the print was the palm print of Lee Oswald then that's the way it would be. How could we know they were lying?....
However ....I'd suggest that you get out your magnifying glass and examine the so called "palm print" on CE 639......If you can find anything but a smudge on that 3 X 5 card, I'd appreciate it if you'd point it out.