Facelifts and model improvements
Fashion changes (facelifts) might have altered very little on the mechanical side, but may have been substantially different in panel work, bright work, trim and body fittings. Additionally in the light of engineering improvements, much might have changed during a production run, particularly in terms of suspension, gearbox, and fuel system. In many cases, we can tell what should have changed in a particular car by reference to the VIN number which Jaguar began using in May 1978, or before that by reference to the chassis number.
World market
A car manufactured for export to say the Middle East will have had a completely different emission control system to one made for the UK, with differences to the compression ratio, fuel system, exhaust system, and body mounting points. The same applies to safety requirements of different markets, affecting bodywork, bumpers, braking and interior safety items such as seat belts and air-bags. Be aware that a car currently in country C may have arrived there via import from country B, but was originally built for country A, with consequent confusion. The VIN number can tell us the original build standard, but it may possibly have been modified for use in another climate.
Post-production modifications
Your car may be quite different to it's original equipment condition due to both deliberate and non-deliberate alterations (i.e. accidents!). Owner-improvements can cover almost anything, but uprating the engine, braking, suspension, steering, gearbox, heating and electrics are all common. Accident damage can result in subtle panel work changes such that when a replacement panel or door becomes necessary, it cannot be accurately mated with the rest of the body. This presupposes the bodywork is actually original - it is not unknown for example, to use a Mk II panel to repair a Mk I car !
Wear and tear
Over very high mileages, you are going to see engine, gearbox and suspension wear, which can often require replacement of parts with a slightly larger size, good examples being piston rings, valves and guides. An expert mechanic will be perfectly aware of this, and we can supply as appropriate. We also draw your attention to the impossibility of matching the exact colour of internal trim, which may have substantially darkened over the years.