Here is a dating chart originally published by Omega:
Note that the date listed for the serial number starting with 7,000,000 is incorrect. It should read "1930", not 1920. That taken with the case serial number of 8.3M means that your watch was likely produced in the very early 1930's.
I think a more accurate name for this case shape would be "tonneau". Someone sold one with a very similar case here a few years ago: