Yeah, that's the one I saw and there's only one "th" in superscript on the page...all the other "th"s are normal size.
It's not enough to find a document from the era with the "th" superscript. There may well have been some even many typewriters capable of printing the "th" superscript in 1968. But the question is, were there any typewriters capable of printing the "th" superscript within the limited universe of machines that could also have produced the document in question? Thus pointing to a document with courier font for example and a superscripted 'th' will prove nothing whatsoever because the document in question was typed by a machine within a completely different category of typewriters.
Am I a typewriting expert? NO, but I did sleep at a Holiday Inn last night.