The only way to do a th on a selectric was to move the typewriter back 1/2 turn and even then the th was large.
Not Selectric. No way. As many posts here have expressed, the IBM typewriter that is capable of proportional spacing is the "Executive," not the "Selectric."
The argument that the document is not a forgery says that teh typewriter was an "Executive" model, and it is not sensible to rebut that by describing what a "Selectric" model can and can't do.
I'm not taking the side that the document is genuine, just pointing out the logic of this particular angle of argument.