So what is the difference between kerning and proportional spacing?
Let me chime in here from my long ago years as a typographer (and by the way I knew Irey and his company).
Proportional spacing accounts for the difference in space that each character occupies on a horizontal line in a typeset line. For example, the character M, which is the widest character is most fonts (along with W), takes up the most space, and would be allocated, in one system, 18 units (an EM space), the maximum. Now the letter "i" as the narrowest in the font, would be perhaps 3 or 4 units wide.
Proportional spacing was not a common feature in typewriters, where all characters were given equal amounts of space in early typewriters. In later years, and with the IBM Selectric series' use of the ball element, limited proportional spacing was available. Proportional spacing is characteristic of typography, rather than common office typewriters. I make the distinction because IBM had a series of typewriters called Composers, which in some ways emulated typography in a limited manner.
Numbers by the way, are traditionally all the same width, or else you could never create columns of numbers that aligned. There are a few fonts where the numbers have different widths, but those font designers have since been severely reprimanded.
Kerning, on the other hand, takes into account the way characters fit together in a manner that is pleasing to the eye. For example, an upper case "W" and a lower case "e" would be kerned, with the "e" sitting nicely under the serif and stroke of the "W", making them easier to read.
I hope this helps.
Proportional spacing is merely assigning as much space as a letter needs to place it in a given area a "W" needs more space than an "i."
In a computer font, kerning is accomplished by making a "kerned pair." "Wi" as a kerned pair would be drawn by the font maker as one entity. When "With" is typed, the word processor program substitutes the kerned "Wi" for the separate "W" and "i." This tucks the "i" in closer and under the rightmost diagonal component of the "W." Makes the word and document more appealing to the eye.
This process was common in typesetting prior to computer word processors, but was simply beyond anything a typewriter could handle.
I am not an expert in any of this, but some years ago had a very intense interest in fonts and different appearing alphabets. I even made several fonts, which I use in my own computer to this day. Spent lots of time reading about how alphabets were made over the centuries, and how fonts have been made since the computer came on the scene.