1967
I don't know that anyone could say for sure what would be correct strap and buckle unless you can find advertising of this exact model. I think personally I would be happy with either a U shield, fat U, or skinny U.
U shield probably the easier to find, although probably a bit early for this watch. I have a bunch myself, but they are all gold plated, nothing in stainless in 6mm size right now.
the only thing I have remotely close is this ladies example, with original strap and buckle and box, but its earlier than yours (this is 59-60'). This has a 9mm strap tapered to an 8mm ss buckle.
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