On the Internet Archive, at
the author of a forthcoming book on the typewriter has uploaded an excellent quality scan (it is a large file, at 900 megabytes, though, I must warn you) of a fairly late Selectric Composer type catalog, thus giving most, if not all, of the typefaces for it.
I have a physical copy of a German catalogue, from which I made a digitisation of Century. It’s included in this collection:
I have wondered about the design of Composer Century, some glyphs like a and g are very un-century-like.
So, basically, they had to design all the different faces to match the same set of widths. This would account for the strange proportions in the a and g of Composer Century. It's a bit like the kind of distortions you get with fixed-width faces. I would guess that the widths they chose were based on what they expected to be the most popular styles, like Press Roman and Univers.
From Heidrun Osterer, Philipp Stamm (ed.): Adrian Frutiger – Typefaces. The Complete Works. 2014.
BTW, said author is Marcin Wichary and the upcoming book is Shift Happens.