Seamaster question?

There were several that can claim to be the first Seamaster. Models 2576 and 2577 were certainly there or thereabouts. It rather depends on how you define Seamaster, since Omega used it purely as a marketing term at first. Despite the launch of the sub brand in 1948, There was no such marking on the watch until at least 1949. I can't find a 2830 before the early 1950s so no, I don't consider that a good candidate for the first Seamaster. By the way the 420 movement used in the 2830 was launched in 1952 according to Ranfft so it is not even close.

The mid 1940s post-war waterproof Omega watches can certainly be considered Pre-Seamasters. The model you refer to is rather later.