As with other Harper & Row series from the mid-1960s, the letter appended as a suffix to the series number in 1965-1966 corresponds to the book's price:
F - $.95
G - $1.25 / $1.35
H - $1.45
J - $1.60
K - $1.75
L - $1.95
M - $2.25
N - $2.45
P - $2.75
Q - $2.95
T - $3.75
It appears that these letters were used only from mid-1965 through mid-1966, and subsequent printings of these titles under the same series number both drop the suffix and have a higher list price. Three travel guides were published on May 25, 1966, out of numeric sequence: CN 301 L (The Greek Islands: A Travel Guide), CN 302 L (Sicily: A Travel Guide), and CN 303 L (Finland: A Travel Guide). See a full-page ad for the guides in PW, May 2, 1966, at 9.
Up until the late 1970s (1978-79), the Colophon editions reliably had an edition line on the copyright page formulated as First HARPER COLOPHON edition published [YEAR]. Starting in 1979, it seems, the hardback and paperback editions of most titles were published in the same year, whereas previously the paperback followed the year after the title came out in cloth. In the absence of the Colophon edition line, the date can be gleaned from the printer's key for the paperback edition, which is usually listed alongside the hardcover edition.
Note: CN 1000 (1972) through CN 1013 (1974) were published out of sequence, but I can't tell why. They seem to be works of psychology, sociology, and political science, although there are a couple exceptions. CN 999 was published in 1983, and the titles from CN 1014 onward follow in sequence. Three Robert Bly books of poetry are out of sequence as well: CN 784, CN 785, and CN 786, which were published in 1985.
The main series appears to have stopped or broken down some time in 1985, when some strange things started happening. CN 1129 (1985), for example, was listed as an in-press Colophon Book in 1984 in PTLA but seems to have been published instead as a Perennial Library book, PL 1129, in 1985. Same thing with what would've been CN 1204, which was evidently published instead as PL 1204.
PW has mentions of Colophon Books in advertisements throughout 1984, including Peter F. Drucker titles (CN 1207, CN 1208, & CN 1209) in the so-called "Harper & Row Management Library," but the last mention of Colophon Books in PW in relation to actively published books is to Novel with Cocaine, by "M. Ageyev" (Short Subjects, PW, Feb. 1, 1985, at 266). This title appears to have been published instead in 1985 as a Perennial Library paperback, 0-06-097000-6.
I'm ending the main series list at #1273. It's difficult to verify many of the titles from late 1985, as far as whether they were published as Colophon Books in their first printing because most Colophon titles from 1985 were reprinted from 1985 through the early 1990s as Perennial Library books. The Colophon line was folded into the Perennial line in the second half of 1985 (PW, June 14, 1985, at 58).
In addition to the main sequence of Colophon Books, there were two separate sequences, CN 4001-4111 and CN 5001-5140. The first ran from 1980-1985 and were all Colophon Books published by Harper's San Francisco office, which had been opened in 1977 (Harper & Row Opens San Francisco Office, PW, Mar. 14, 1977, at 46). In addition to general works, the San Francisco office also seems to have specialized in religious books. The second run, CN 5001-5140 ran from 1974-1985 and consists of books published by Basic Books, which Harper had purchased in 1969.
Harper Colophon Books: Fall Paperback Series, Publishers Weekly, May 28, 1962, at 43-44.
Ad, Publishers Weekly, June 4, 1962, at 16.
Eugene Exman, THE HOUSE OF HARPER: ONE HUNDRED AND FIFTY YEARS OF PUBLISHING 298 (Harper & Row 1967):
In the field of supplementary or assigned reading in the universities, paperbacks have made and continue to make an important contribution. Material of this sort is supplied by Torchbooks and Colophon Books, the two quality paperback lines, with presently almost 700 titles available, the bulk of which are reprints of established scholarly works in various fields. Perennial Library, the latest paperback line, is lower-priced and aimed at school and college students with the emphasis on literature. Cass Canfield, Jr., is the publisher of these three paperback lines, which for the next fiscal year should produce net sales of nearly three million dollars.
Cass Canfield, UP AND DOWN AND AROUND: A PUBLISHER RECOLLECTS THE TIME OF HIS LIFE 231 (Harper's Magazine Press 1971).
It was during these years that Harper's became preoccupied with starting its own line of paperbacks to supplement our indirect interest in paperback publishing through stock ownership in Bantam Books. Melvin Arnold, then a religious-book editor under Eugene Exman, undertook to develop Torchbooks. an attractive-looking and carefully chosen series designed to appeal largely to college and graduate students. Torchbooks achieved immediate recognition and success and two other paperback lines were soon added: Perennial and Colophon, both of them aimed at a less scholarly and younger audience than Torchbooks. Although Perennial is now a fast-selling line, it ran into serious difficulties after a year or two due to overproduction, and when Arnold assumed general management responsibilities for Harper's, Cass, Jr. was chosen for the difficult task of getting our paperbooks back on the rails. He achieved this with firmness and skill so that Torchbooks, Perennial, and Colophon books, the three Harper paperback lines, are now very profitable.
Harper & Row to Consolidate Paperbacks, PW, June 14, 1985, at 58 (announcing the effective end of the Colophon line, whose titles would be rolled into the Perennial line. Also noting that the Colophon line "was launched to publish books for sophisticated general readers who bought their books in bookstores.")