Is it true that Selectrics printed proportional fonts? I don't see how, since they used a print ball. Seems like coordination of movement of ball across page would be difficult. Could be wrong.
The basic task of the IBM Composer was to produce justified camera ready copy using proportional fonts. It has the capability of using a variety of font sizes and styles. The first IBM Composer was the IBM "Selectric" Composer announced in 1966. It was a hybrid "Selectric" typewriter that was modified to have proportional spaced fonts. It is 100% mechanical and has no digital electronics. Since it has no memory, the user was required to type everything twice. While typing the text the first time, the machine would measure the length of the line and count the number of spaces. When the user finished typing a line of text, they would record special measurements into the right margin of the paper. Once the entire column of text was typed and measured, it would then be retyped, however before typing each line, the operator would set the special justification dial (on the right side) to the proper settings, then type the line. The machine would automatically insert the appropriate amount of space between words so that all of the text would be justified.
http://www.ibmcomposer.org/SelComposer/description.htm
Doesn't sound like TANG equipment...