I can’t speak for the 1970s but in the 1980s…
- Cinelli 1/a steel expander bolt replaced with aluminium from 1/r
- short piece of dowelling into fork steerer to supposedly reduce brake shudder and/or for crown/steerer failure contingency
- Ill-advised tubular selection including Clement Campionissimo Seta Extra and Nuovo Super Seta Extra for road races on sketchy roads
- seat post cut short for minimum insertion into frame
- 2/3/4/5cm cut off end of handlebar drops
- aluminium crank arm attachment bolts requiring initial steel bolts to tighten before substitution
- Keywin pedals were everywhere in New Zealand in the mid-1980s, probably half the weight of Campagnolo Superleggera
- Modolo Kronos plastic time trial brake levers used on road bicycles. They were almost purely decorative.
- Ofmega Mistral plastic rear changers and shift levers.
- very occasionally a steel fork replaced with a Vitus 979 or SR Litage aluminium fork, although Alan/Vitus aluminium/Carbonio/Carbone 3 and 9 frames were pretty common.
By the 1980s there was only so much left to do because racing bicycles had largely evolved down to the bare minimum needed for functionality by then.
I did hear of racers removing a few loose bearings each from headsets, hubs and bottom brackets but never saw it first hand.
Good times.
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Nothing quite says wall hanger like drillium toe straps