I believe the only way we can answer this question is by members finding 'the earliest' such description.
DEC's first CRT terminal device, the DEC VT05 terminal (1970), had tab stops fixed at 8-character intervals.
The DEC VT50/VT52 terminals (1974) had tab stops fixed at 8-character intervals.
The DEC VT100 terminal, (1978) which replaced the VT52, had tab ...
[No time for a real answer (got to push my narrowboat from Wolverhampton to B'ham), but ...]
This page (which is a marvellous collection of TTY manuals) does have at least two charts (7171WD, 7172WD) showing their encoding/population side by side (caution, strange PDF settings).