"451" is only part of the serial number......the serial number for this rifle is 451 over 1916, followed by "LE". There were heaps of Mk III's made and issued in 1916 using number "451" and "1916", but only one of them was part of the other 998 rifles in the "LE" series.
An extremely clean example indeed, but from what I see, I think "Chislerboy" (could he be the son of Saint Cliff of Chisler?) is right on track regarding creative stamping....