Well, Donnie, I am just a bit slow at reading diagrams.
After I seriously started looking for the tstat, I found it easily.
It is simply 90 degrees off from where I expected to find it ... it is on the left (facing the engine from the front) side and does indeed connect to the lower radiator hose.
Success story, though ... I replaced tstat and the viscous clutch and I am back 4.0 ... I tested the old tstat in heating water and watched it ... seemed to be OK, but don't know if that is a suitable test any longer, particularly with this tstat.
The car now warms up faster (my imagination?), stabilizes at 82-85C and moves up only in response to load and heat and a/c use ... all very predictably. I was unable today, in the hottest weather this year in WDC area (98F), to get it above 98C or so ... would not touch the 100C mark - which is the way it was before I worked on it a month ago.
Now, the question is: is a tstat more likely to fail while the coolant is drained? The only thing I did a month ago was replace the coolant pump and the viscous clutch (was the replacement viscous clutch the issue? ... probably not since the temp went up even while cruising at 75 mph and there is more than enough ram effect to keep it cool at that point, I think).
Anyway, thanks for your comment and help!
1991 350 SDL (200K and she ain't bent, yet)
former 2002 E320 4Matic Wagon - good car
former 1985 300 CD - great car
former 1981 300 TD - good car
former 1972 280 SEL - not so good car
a couple of those diesel Rabbits ...40-45 mpg