Pennsylvania State Parks
Pennsylvania does have a permit process for our state parks and forests. I have done it and it is very easy and have had no problems with it. You can find the form here:
It is the geocaching form, but it has a checkbox for letterboxes as well. Simply attach your clues and also a map marking where you plan to place your boxes and turn it into the park manager. It should take about a week or two for them to approve or deny the permit. If approved you will be given a sticker to place on the box to mark it has been permitted. The permit is good for three years.
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