The lights moved to the wings in 1968, I think, and was due to US legislation (US exports were still big enough then to warrant making the mod standard across the whole of production. It gives better lighting but many people prefer the original aesthetic and the grille location was better protected from impact damage.
The photo above shows an early II/IIA grille panel with lights and very slightly later IIA wings with "bug eye" lights, which were fitted to unmodified front panels. The panels were soon redesigned with a flat recess and reinforced mounting for less protrusive lighting, and the exact design for those IIA wing fronts is unchanged to the latest Defenders. The combination shown is, indeed, an owner-mod. I have seen it done much more smartly with the same grille and the later wings by a Californian lad on another forum (Greg, I think his name is).
There are a lot of interchangeable parts, but not much from a SI (1948-1958) looks similar or fits a SII or later vehicle. However, much from an early SII will fit a Defender and vice versa. Commonality between a SIII (1972-1984) and the Defender is even better - I have Defender doors, jerrycan lockers, bonnet, grille, windows and front wings on my SIII.