Some change points from 'The Complete Mini' by Chris Rees:-
October 1970 to Jan 1971 - Steering lock introduced across range.
1971 - Radiator cowl deleted.
Dec 1972 - Alternator standard.
Pre 1973ish.. rear seat bulkhead, cars should have the big 'rotodip' hole.
April 1973 - inboard driveshaft pot joints, rod gearchange with larger front floor tunnel.
June 1973 - improved door check pivot brackets.
Feb 1974 - inertia seat belts standard.
May 1974 - SU HS4 carb (850)
June 1974 - HRW standard and twin silencer exhaust.
Pre 1975 cars have shallow dip in front bulkhead cross-member to clear carb, later dip is deeper to clear later type of SU carbs.
October 1975 - Anti-tip seat catches, seat belts mounted in rear pockets. Cars made prior to 1975ish.. had welded-in ashtray holders.
May 1976 - Twin column stalks, new rocker switches,hazard lights standard, larger pedals from Allegro, moulded carpets, steering lock from BL Princess, softer rubber cones, new subframe mounts 1976 onwards (Mk4) cars front subframe have one single big bolt with a rubber mount instead of twin fixing bolts. 7.5 gallon tank
July 1977 - Padded steering wheel and handbrake grip from Austin Allegro
May 1980 - 'Silent Mini' sound proofing added, perforated headlining.