Nacha
The body that owns the rulebook for the US ACH network — and the fixed-width file format that still moves trillions of dollars a year.
Nacha, originally the National Automated Clearing House Association, was formed in 1974 when regional clearing associations joined forces. It runs no network itself — the Federal Reserve and The Clearing House do that — but writes the rules: which return codes exist, how long a receiver has to dispute a debit, and what it takes to be allowed to pull money from an account.
Its most visible legacy is the NACHA file: fixed-width records of exactly 94 characters, padded with spaces, in a layout that looks like it came from the punched-card era. It did. Same Day ACH was phased in from 2016, and the file format survived that too.