It is supposed to be tightened by tapping it counter-clockwise, same direction as the rotation, looking at the PTO. So, theoretically, it should be loosened by tapping it clockwise. With about a 3/16 drift, and a 16 oz or better hammer. Give it a good whack.
Caveat is that it is probably just as often tightened the other direction, so if it doesn't come loose CW, give it another whack CCW. Good luck!
And welcome to OCC!
Oh, the reason it is tightened CCW is because with CCW rotation, force on the PTO is CW (resistance to rotation) which would tend to tighten the bearing inside the eccentric lock collar.
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