My 68 222D has a 2 piece prop shaft. BUT, I have a reference manual called "Mercedes Benz 220/8 1968-1972 Owners Workshop Manual" which clearly says that both 2 piece and 3 piece shafts were available in these cars. It unfortunately doesn't say what determined this design choice. If you trust the diagram in this book showing these two options, both are for the same length of car. In the 2 piece, the first one is small diameter and the second is larger. In the 3 piece, all three pieces are small diameter. Not sure if any of this is relevant, but I wanted to share.

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1968 220D, w115, /8, OM615, Automatic transmission.
My 1987 300TD wagon was sold and my 2003 W210 E320 wagon was totaled (sheds tear).