My newly acquired 2852 Connie has this crown, can anyone enlighten me as to the extra mark inside the Omega logo? My guess is a mark indicating a waterproof crown?

 

This is a Naiad crown and can be found on Constellations with the Naiad case.
It's also found on the 2913 SM300 and the 2914 Railmaster models.
A bit more info: http://chronomaddox.com/three_high-precison.html

 

I think it's called the 'mercedes' mark, Omega used to put it to indicate waterproofness.

 

Which, it turned out, it was not, generally.

OOOPS