[AlterSchinken]

That's a casting number, not a part number. Are there any other numbers stamped anywhere? Where exactly was it located?

 

[dvanpelt]

My '75 450SL had a plastic cover like that over the crankshaft pulley. I had to remove it to turn it over manually while timing it.

 

[Jeff Weiss]

My '75 450SL had a plastic cover like that over the crankshaft pulley. I had to remove it to turn it over manually while timing it.

Yup. That's what it is!