baffling Omega 2494-6, 2657

Hi all,
I just recently purchased a vintage Omega case with both designations: 2494-6 and 2657 that looked in a very good condition. I had both a 351 and a 354 movement with dials that are supposed to fit this case. However, both dials seem and measure too small for this case even though I have seen similar dials on some pictured watches. Both dials measure at 27.5mm but the placement for the dial in the case measures at 28.5mm. Am I missing some kind of a filler ring / inside holder bezel, or my research on the size of the dial was wrong to start with? I can use the help if anyone knows the right answer to this question. Thank you in advance.

Thanks. By the way do you know what the actual measurement for that case's dial is? I have not come accross a single dial from the period fitting 351 and 354 movements that are a different size.

I don’t know the dial size, but the vast majority of Seamasters of this era were the normal 34.5mm size, presumably with the dial size you mention, not the oversized 36mm size of this case. So it makes sense the larger dial would be more rare.

Thank you so much for the detailed answer. I guess I will keep looking.