As my mind wandered a bit, this a.m., I finally sought the answer to a couple questions I've had since beginning my MB diesel journey. I'm sure many on this forum already knew this, but, maybe there are some newer members, such as myself, that didn't......
OM606
The abbreviation OM means Oelmotor (oil engine), and stands for a Daimler-Benz engine that uses any kind of light fuel oil as fuel (diesel engine).

W123
W means “Wagen” cart. Karre, Wagen, Karren, Kofferkuli. car.

I feel better, now. Life's simple pleasures.