Franken or not? Omega Seamaster Calendar

No its not all original. The caseback says 2846/2848 and is for a non date watch. Note how someone has crossed out the last digit and scratched in a 9? Something has been swapped, maybe just the caseback.

Here is the OVD info for what you actually have there:
https://www.omegawatches.com/en-gb/watch-omega-seamaster-calendar-ck-2849

Also, the movement looks like it could use a lot of love. And whoever tried opening the watch wasn’t particularly skilled, it seems (might be just the corrosion misleading me there, though).

Looks like a 2849 with a wrong case back

Why would you spend your money on something that clearly isn't correct and in poor condition as well?

I don’t think this is a bad price for a nice looking 1950s Omega. At that age you expect a few wrinkles. And as @schoolboy once said, “no one sees the back”.

Of course, you have to factor in a service but it’s a quality watch with a quality movement and, although you clearly won’t ever sell it on to an OF expert, you’ll enjoy wearing it, in due course someone, somewhere will be happy to take it off your hands for a reasonable price and, whilst you may well get a nicer Calendar at bigger price, you may not and you might regret passing up on this. Still, your call.