While 74 was the first year for the big bumpers, there are also a host of changes for 73, that differentiate the 73 from the earlier cars.

To answer two of your questions:

Interior. To do it right, everything needs to be changed... Different grain on the upholstery and trim, different wheel, etc.... 72 and earlier uses a leather grain, 73 and later, a pinpoint grain. Classic hard plastic wheel with horn ring changes to a urethane wheel in 73. Vent window and mirrors change in 74.

Headlights. You mean the headlight doors, right? Headlights are industry standard 7" sealed beams. Headlight doors have an early and late version. Very early version has an amber lens for the parking and signal light. Later version has a clear lens with an amber lens behind the clear outside lens. Change occurred around 69.

Jim

__________________
14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles