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khj, I think that is true for the souvenir sheets. The note in Scott states it for a fact that single sheets can't be distinguished. Only the pairs and blocks showing the gutters can be positively identified.
Yes, #769 and #770 are S/S, and my comment regarding not being able to distinguish from the original printings applies only to S/S.
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I do have the national parks inperf. Is it your understanding that the imperfs are Farleys?
As a general statement, that is true for the National Parks imperforates (#756-765).
However, some of the original printings (#740-749) are known to exist as imperforate errors. Since PO agreed to gum any Farleys that were sent in, well... you can guess the problems with that. Just like when the PO deliberately issued the Hammarskjold invert.
Any #740-749 imperforates should only be purchased with a certificate, as the original gum is often indistinguishable from the gum on the Farleys. The certification is often based on known history of the item, more than anything else.