I just discovered an old notebook from the 1960s, probably my first collection of notes on constructed languages other than Esperanto. The first section begins with a photocopy of Appendix II: A Tentative List of Artificial Language Projects, from Mario Pei's One Language for the World, followed by my handwritten list of 145 projects mentioned in the book, dated 24 June 1969. I compiled a list of 20 Latin-based projects, followed by a varied list of 31 projects. Then I made a list of philosophical a priori language projects: Dalgarno, Wilkins, Leibniz, Lodwick, Beck, Gibson's Code, Perio. The last two are not from that era. The next list includes samples: Langue Universelle et Analytique (E. Vidal, 1844), (Delmore, 1795), (C. Letellier, 1852), (Sotos Ochando, 1852/5), Timerio (Tiemer, 1921), Zahlensprache (Ferdinand Hilbe, 1901), (Dalgarno, 1661), (Wilkins, 1668). In addition to Pei, sources include Drezen, Monnerot-Dumaine, and Pankhurst. I've got a couple pages apiece on Suma (Dr. Barnett Russell, 1957) and Solresol. Then there is another list with details of 42 projects. Then I have a page listing the scanty details of the two projects of reformed Hebrew: Pasilingua Hebraica and Lasonebr. Then there are separate descriptions of 1-3 pages apiece of Idiom Neutral, Occidental (both stamped 12 July 1969), Romanal (stamped 14 & 19 July 1969), Volapuk, and Interlingua (new).
I have whole sections devoted to Ido and Latino sine Flexione (old Interlingua) (with comparisons to Romanal et al). Sources include Pei, Pankhurst, Guerard, Jespersen.
The next, thick section is on Neo, from A. Alfandari's Rapid Method of Neo, followed by another section, on Ro, from Alphabet of Ideas or Dictionary of Ro, The World Language by Rev. Edward Powell Foster (Waverly, WV, 1919, 1928), stamped 18 July 1969. (I think Ro was not discussed by Okrent, oddly, since it gained a fair amount of attention in its day.)
The next section consists of extensive notes from E. Drezen's Historio de la Mondolingvo, going as far as Spokil (1889-1904). There is a section on Mario Pei's One Language for the World; it looks like all these notes were copied elsewhere. The next section consists of a few photocopied pages from E. Sylvia Pankhurst's Delphos, The Future of International Language (London, 1927), some pages of which can be found in other sections. The final section is labeled "Superfluous or Duplicate Notes": the only notes there are on Solresol.
Now back in those days conlangs were not "conlangs", as they were not considered to be languages created only as hobbies or for literary/fantasy purposes (with the exception of Tolkein's Elvish and Dwarvish), even though this subject matter was a hobby for many. This whole series of language projects fit the first two eras of artificial language creation: the philosophical languages associated with the scientific revolution, and the international auxiliary languages (of which Esperanto is the most successful). I read all the major surveys which could be found then (in English, plus Monnerot-Dumaine's book in French). In addition I investigated specific projects for which there were separate books, ranging from Novial to Babm. I'm guessing that much of this material has been deaccessioned from the main Buffalo and Erie County Public Library, which was a treasure trove of this old and obscure material (and with some old and rare Esperanto publications as well). This notebook is not all I have, but it was a major manifestation of the way I organized my notes into three-ring notebooks at the time: this one was designated "E1".
What was once an adolescent hobby for a handful of male nerds has mutated and proliferated into a socially acknowledged phenomenon with an entirely different orientation, i.e. the era of conlangs. How different the world is in so many ways from the world at large and my small world of 1969.