How do you know it is unfired?
Would a buyer be convinced.
If it really is unfired I would keep it that way.
Better yet, sell it and buy one that will not go down in value from shooting it.
I would take the trigger guard off and look at the little piece of wood between the magazine and the trigger cut out. If it's cracked it's a good bet that it's been shot. I've never taken a P64 in 458 apart but I've taken quite a few P64 375's and have yet to see one that hasn't been cracked there. Note that none of the 375's that I've examined were ever in question for being unfired. The crack doesn't hurt anything but there just wasn't enough wood there to take the recoil of the bigger calibers.
