You must be confusing these with something else. Every Selectric had a fixed carriage and moving head.

IBM Selectric: https://www.youtube.com/watch?v=vNUEUth7qjc

IBM Wheelwriter: https://www.youtube.com/watch?v=ZabsF0aIE6k

NEC Spinwriter: https://www.youtube.com/watch?v=tCCB0AW9yT4

Diablo: https://www.youtube.com/watch?v=C5bodpSsFl8&t=342

Teletype Model 33: https://www.youtube.com/watch?v=6N8gu5F894k