...the seats will interchange.

As long as both the donor seats and your car have the same type of headrests, i.e. power to power or manual to manual, all seats interchange.

Worst case is that you need to remove the six fasteners for the power tracks and keep your car's power track for the new seat.

Variances on the W126 series consist of additional features as the cars gets newer. Early cars had manual headrests, and no memory. By 86 both sides had memory and power headrests. I recall 85's had the power headrest, but don't remember if both sides had memory.

Jim

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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