This is an incredible coincidence! About two weeks ago I actually picked this Seamaster DeVille at another Salvation Army, although I did not realize quite what it was until I cleaned it off and polished the crystal a bit. Lacking the Seamonster on the back, I thought it was something else. I now have my first Omega, as long as it is an original!
Your much more knowledgeable minds would correct me, but I expect to find (after employing my Bergeon crystal lift and your advice as far as the stem) a 550 or 552. Am carefully familiar with crystal lifters and stems because of my affliction with Benrus DTU2A/Ps and early Hamilton GGW113s.... Beyond the serial number, what else should I be looking for here? Most concerned with careful preparation and that it may be a fake inside. Fortunately, the Salvation Army price was not one to worry over.
It's set for a cleaning and service afterwards! It will need a new second hand and will replace the worn Speidel band with a nice leather one.
Finally, thanks to all of you for some very informative and useful advice. Since finding this particular watch, I have lived on these sites.
Matt