18981 is unique. First of all, the sum of first or last two numbers equals the middle number. Also the sum of the last three numbers equals the first two numbers. Turn it upside down and it's 18681 and makes time run sdrawkcab. Add the first two number and multiply by the third and the result is the last two numbers. AND while it's palidromic, it's not a prime (3 x 6327) but there is only one way to obtain it, as I've demonstrated). If, however, you add all the numbers together you get 27, the sum of which is 9, or 1 + 8, or 8 + 1, or the middle number itself.

There is another thing about it, but not even Amos will be able to figure out what it is! Ha ha ha!