The standard allows round unthreaded, 10-32 threaded, and 12-24 threaded holes. It does not specify the modern "square hole" cage nut pattern. Square holes became popular through de facto industry adoption because they accept both metric (M6) and imperial (10-32) cage nuts. However, square holes are technically a later evolution (often referenced in EIA-310-E).
In the early days of digital communication, devices from different manufacturers couldn't communicate with each other seamlessly. This led to a lot of confusion and frustration among users, who were unable to connect their devices to networks and exchange data. To address this issue, the Electronic Industries Alliance (EIA) formed a committee to develop a standard for data terminal equipment (DTE) that would ensure interoperability and compatibility. eia310d standard pdf