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While typewriters did produce very good letters, in the 1960's, they did not yet have carbon ribbons. There would be some variation in ink distribution from one point on a page to the next, even if both were produced by the same type element. This variation would be based on the thread placement in the cloth ribbon.

Even a "carbon copy" made with carbon paper would be affected by this because pressure for transferring the carbon would vary slightly with the ribbon threads. You can see this in old carbon copies made on typewriters with cloth ribbons. You get a bit of the ribbon fabric impression even in the carbon copies, plus some effects from the fiber of the first layer of paper.

Finally, unless the mechanisms of the typewriter were absolutely perfect, and the scanner was set to an exact division of both the vertical spacing and horizontal spacing, there would be some difference in the way the scanner handled the edges and "antialiasing" of the same letter from the same type element on the same page. For example, at 100dpi, 200dpi, 400dpi, most typewriters have spacing which would make the same letter on different lines or in different horizontal positions on the same line scan differently. If there's any wear in the typewriter mechanism(s), even scanning at something that was an exact match for vertical and horizonal spacing (possibly 150dpi, 300dpi or 600dpi) would show anamolies from spacing variations from the wear.

I haven't repeated the experiment in the youtube video. However, identical pixels on the scan in two different positions on the paper would be very suspicious. It's difficult to explain other than digital modifications (copying) after the scan.

I'm figuring the typing actually was done in 1961 in Hawaii ~ on two different typewriters set to identical tabs. The quality of the letters suggests they were manuals ~ one maybe a bit newer than the other.

Now, about the ribbon ~ I sure don't recall carbon ribbons at the the time but I do remember using old, faded, stretched and well beaten down cloth ribbons that'd be re-inked.

All things are possible. I suppose all sorts of woven materials were used for ribbons at that time.

None of the typing is at carbon ribbon levels but there are pre-printed items on the page. Thankfully we are past that point in time where folks were taking a look at the rubber stamp images and telling us it was fake since those characters didn't match that of the typing.

Not a fact: "While typewriters did produce very good letters, in the 1960's, they did not yet have carbon ribbons."

The FIRST typewriter ribbons were strips of carbon paper! Carbon paper was invented in 1808. Practical typewriters came around sixty years or so later and they used carbon paper ribbons.

See http://www.kevinlaurence.net/essays/cc.php