If the likes of 1940 are, as in your sheet, text then:
=IF(ISNUMBER(A2),TEXT(A2,"MMMM d, yyyy"),TEXT(A2,"@"))
?


If these aren't text, then 1940 gets converted to 23 April 1905, if this is out of scope for your possible dates you could set a limit say 2050 or 3000 (respectively equivalent to 11-Aug-1905 and 18-Mar-1908) and test if it's above that limit:
=IF(AND(ISNUMBER(A2),VALUE(A2)>2050),TEXT(A2,"MMMM d, yyyy"),TEXT(A2,"@"))
Problem now might be that a blank cell gets converted to a textual zero which can be circumvented with:
=IF(AND(ISNUMBER(A2),VALUE(A2)>2050),TEXT(A2,"MMMM d, yyyy"),IF(TEXT(A2,"@")="0","",TEXT(A2,"@")))