I started in 1977 programming FORTRAN on an ICL 1900 mainframe running GEORGE III, a brilliant o/s. Without exception everyone I have spoken to who used it says it is the best o/s they ever worked with. The mainframe was 250 miles from the office and we had a teletype and a 100 baud acoustic coupler, sort of like a modem but you dialed the computer with a normal phone and then put the handset into rubber cups on the coupler once the connection was made and it listened to and translated the funny noises.
The mainframe had 128k words of memory and a word was four 6-bit "bytes" so there were no lower-case letters in the character set. Also, the numbers 0 to 9 were represented by 000000 to 001001 so their character was the same as their value. This caused me no end of confusion when I moved to ASCII machines.