What is this tag on my 616?
 

see photo

I believe it means that you have a factory-rebuilt engine. This thread explains "Tauschaggregat":
http://www.peachparts.com/shopforum/vintage-mercedes-forum/345987-engine-id-tag.html

Cool. explains why this engine is so good.

One of my dad's SD's had the same, and that engine was awesome! Had a fair bit more grunt than the others we've owned. Probably just younger/higher compression. It once even started in sub 30 degree weather with 3 dead glowplugs with about 40 seconds of cranking....super strong motor.

On a drop in engine. If you have the smaller glow plugs on the rebuilt. The 240d engines where improved later in their run and have a little more power as well.