From 1981 to 1985 The 300SD (126) had the 617.952 Turbodiesel, 5cyl.
From 1986 to 1987 the 300SDL (126) Had the 603.9?? Turbodiesel, 6cyl 3.0L.
From 1990 to 1991 the 350SD/SDL (126) Had the 603.9?? Turbodiesel, 6cyl 3.5L.

Hope this clears things up.

PS: Thought the W116 turbo motor was slightly different, I thought it had 115HP or 118HP, rather than 110.