Hello:
Here is a fairly large amount of info about the early days of the Elo rating system and its development. I hope someone can enjoy the reading!
------------------------
There is a progress report by Arpad E. Elo called
'Progress report on rating systems', published at different issues of
Chess Life in 1960:
https://uscf1-nyc1.aodhosting.com/CL-AN ... 0_03_1.pdf [Part I].
https://uscf1-nyc1.aodhosting.com/CL-AN ... 0_04_1.pdf [Part II].
https://uscf1-nyc1.aodhosting.com/CL-AN ... 0_05_1.pdf [Part III].
------------
More about the then new Elo rating system was published at the June, 1961 issue of
Chess Life magazine:
https://uscf1-nyc1.aodhosting.com/CL-AN ... 961_06.pdf
According to the article, two additional publications were available to students interested in the system and its application: an 'Analytical Supplement' and 'Examples of Computations', if asked to Elo himself.
------------
Elo rating system appeared again at different issues of
Chess Life magazine:
https://uscf1-nyc1.aodhosting.com/CL-AN ... 962_08.pdf [August, 1962].
https://uscf1-nyc1.aodhosting.com/CL-AN ... 964_04.pdf [April, 1964].
https://uscf1-nyc1.aodhosting.com/CL-AN ... 967_08.pdf [August, 1967].
------------
Additional two-part explanation:
https://uscf1-nyc1.aodhosting.com/CL-AN ... 971_02.pdf [Part 1. USCF Rating Regulations].
https://uscf1-nyc1.aodhosting.com/CL-AN ... 972_03.pdf [Part 2. How We Calculate the Ratings].
------------------------
Elo rating system replaced Harkness rating system.
Chess Life dedicated a series of articles in 1952 called
'How the Rating System Works' to explain the Harkness system:
https://uscf1-nyc1.aodhosting.com/CL-AN ... 2_05_2.pdf [Part 1].
https://uscf1-nyc1.aodhosting.com/CL-AN ... 2_06_1.pdf [Part 2].
https://uscf1-nyc1.aodhosting.com/CL-AN ... 2_06_2.pdf [Part 3].
https://uscf1-nyc1.aodhosting.com/CL-AN ... 2_07_1.pdf [Part 4].
https://uscf1-nyc1.aodhosting.com/CL-AN ... 2_07_2.pdf [Part 5. Round Robin Performance Ratings].
https://uscf1-nyc1.aodhosting.com/CL-AN ... 2_08_1.pdf [Part 6. Rating Short Tournaments].
https://uscf1-nyc1.aodhosting.com/CL-AN ... 2_08_2.pdf [Part 7. Rating Swiss System Tournaments].
https://uscf1-nyc1.aodhosting.com/CL-AN ... 2_09_1.pdf [Part 8. Rating Swiss System Tournaments (continued)].
------------------------
Disclaimer: this list might be incomplete.
Regards from Spain.
Ajedrecista.