In the 70s I believe that there was a two hourly service loco hauled (Class 33 before 50s took over) between Waterloo & Exeter, reasonably clock face in the down direction, less so Up. In addition there was a two hourly DEMU service betwen Salisbury & Reading with a connection to Waterloo at Basingstoke. There were additional peak services to and from Waterloo - there was a hybrod traction 33/1 + 4TC + 8 Vep (or Cig, I forget) that left Waterloo around 18:05 and possibiy extra trains west of Salisbury.
As a matter of interest the 33/1 +4 TC worked a couple of Waterloo - Olympia shuttles before their Salisbury working (no Overground in those days).
My memories is rather hazy though - it was 50 years ago.
The typical 80s timetable was xx10 out of Waterloo, alternating between Exeter and Salisbury. The Exeters were 50-hauled + earlier Mk-II and departed on the odd-hours, retaining the steam-era pattern of odd-hour services on the hour.
The Salisburys used 33s and many were push-pull with 4TC or 8TC and left on the even hours. In addition, one or two workings used the Southampton boat train sets (there's a discussion on this from around, IIRC, October 2024 on here somewhere).
Up services were much less clockface, constrained by the single-line working and presumably conflicts at Exeter.
In the 70s the 33-hauled services left on the hour odd hours (the classic steam-era pattern), I believe the move to 50s occurred in 1980 and led to an acceleration hence the move to xx10.
1991 had an erratic service mostly less than hourly but with a significant peak ramp-up; from my understanding there were some reliability problems in this period. Between 0900 and 1900 there was: 1000 Salisbury; 1100, 1315 and 1515 Exeter; 1615 Yeovil; 1655 Exeter; 1730 Yeovil and 1815 Exeter.
When 159s came in (May 1993) the pattern moved to xx35 but still with the alternating-hour service. Even hours went to Exeter or occasionally beyond, odd hours terminated variously at Tisbury, Gillingham or Yeovil.
At some point between 1999 and 2004 an additional xx03 service to Salisbury was added every hour. Some of these were single 2-car 170s. By the Dec 2003 timetable, the xx35 left at xx33 and mostly remained 1tp2h to Exeter in the even hours and Gillingham/Yeovil terminators in the odd hours, though the 1133 extended to Plymouth.
The recent (1994 onwards) timetables can be found via Network Rail working timetables (section WG) downloadable from Network Rail. 1982 is available from Timetable World as a free download of the GBPTT as are 1965 (?) and 1973 within Western Region timetables.