These front loading (unishell, monocoque..... there were several names for this case style) automatic Seamasters didn't carry the word DEVILLE on the dial until 1963-ish. Before then, they just had Seamaster. The case was introduced in 1959-ish and switched from a 5 digit reference number to a 6 digit reference number in 1962-ish like nearly all Omega numbers did then. The only time I saw DEVILLE on a dial before 1963 was if it was a manual wind 600 series caliber inside, and they weren't all that common.
It's certainly a 56x inside because of the date window.
BTW, I might be off by a year.
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