That is a very beautiful car. I've seen many 300D's with leather. The plain, as in non pleated sections were not used on w123 with leather. The style with the plain panel borders was used on 126, 201 and 124 series till they were refreshed in the late 80's. On those models it is a very quick way to discern. On older 116, 123 and 107's you can discern by texture. It takes a little practice to be able to tell quickly, just from viewing pictures.

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1979 240D, 4spd manual, Power Sunroof, manual windows, 147k miles, Pastel gray/Black MB Tex.
1991 300D 2.5 - Smokes like it's on Crack!