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The answer lies in the difference between those keys. scaleFactorOfFirstFixedSurface and scaledValueOfFirstFixedSurface are integer keys with 1 and 4 bytes respectively. Their values can be any integer that will fit in that many bytes, for example scaledValueOfFirstFixedSurface can go from 0 to 4,294,967,295 (=2**32 - 1). Setting it to the maximum value will set all the bits to 1 which is the MISSING value (WMO Regulation 92.1.4). So when you use the keyword "missing" in the first command, ecCodes actually sets this value.
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