In answer to the first question, Not Likely , as for the second part over the years I've sometimes seen GPC's with no postmark or cancel stamp but they are usually addressed, as what would happen is collectors would send their GPC to be signed and the signer would take it upon themselves to mail the item back in an envelope usually with a letter and/or photo.
BUT a cancel stamp address & postmark on a GPC does not insure authenticity either as I have seen those offered as well , what they usually have in common is they are always in mint condition , never inscribed and the seller or sellers never seem to have any of the low end or non-hall of fame signatures to offer , Only expensive hall of fame names. Hope this was helpful.
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jim@stinsonsports.com