Thanks to the grainy screen-grab below (courtesy of AHSchwartz on Twitter), we can deduce that the sign went missing between July 6, and July 17.
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We have already dispatched the Hardly Boys to investigate this heinous act. In the meantime, take a look at the numbers that Mickey Mantle put up in 1952 (his age 20 season), to help lead the Yankees to their 4th consecutive title. Wouldn't it be nice to watch a 20 year old put up numbers like that in the big leagues? What would Jesus (Montero) do? Too soon?