Question about "First Strike" on material in a mint sealed box

How does PCGS determine "First Strike" on a mint sealed box when there is no date on the outside of the box. Do they use the date on the packing slip and not a delivery date? I have some material that I know was ordered/delivered within the first 30 days that would qualify for "First Strike" but there is nothing on the outside of the box to indicate the date.
I could always go online and print out the order showing when it was ordered (prior to the 30days) and the matching tracking number. I do this as a practice so I know what is in the box (tape it to the outside of the box). Not sure if they would accept that as proof for "First Strike" or not.
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Comments
Date is on the packing slip
Thanks for confirming, that was my original thought but wanted to make sure.
PCGS website explains it all
Good information !!!