IBM typewriter with proportional spacing was introduced in 1941.
IBM announces the Electromatic Model 04 electric typewriter, featuring the revolutionary concept of proportional spacing. By assigning varied rather than uniform spacing to different sized characters, the Type 4 recreated the appearance of a printed page, an effect that was further enhanced by a typewriter ribbon innovation that produced clearer, sharper words on the page. The proportional spacing feature became a staple of the IBM Executive series typewriters.
On the Executive, you could optionally have removable type-bars. This is somewhat like later Smith-Corona portables which have removable type-slugs on the two outermost type-bars, with corresponding changeable keytop caps. In this case, though, it's the whole type-bar.
I've got one of those, exactly like the one in the old advertisement you posted. I bought it at an auction of Texas state agency used and surplus office equipment in 1980 and lugged it around all through college and law school. The thing weighs a ton but was built like a tank.