The SL clocks were completely mechanical and mounted in the glovebox door.

Opening the door, you could see the winding wheel, which when wound, gave you a week's worth of clock. Clock was not illuminated when dash lights were switched on.

Yours is different in the following ways:

Number font is incorrect for SL Benz.
Plug is for what? Light, Electric wind? Or both?
Case looks like it's a universal ex post facto factory mount style, i.e.: aftermarket accessory.

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