How Come Victor McLaglen's Character Had No First Name?

The three main characters in Gunga Din are identified as MacChesney, Cutter and Ballantine. However during the film we learn that Cary Grant's character's full name is Archibald Cutter, an inside reference, as Grant was born Archibald Leach. We also learn that Douglas Fairbanks Jr. is named Thomas Ballantine, actually we even learn his middle name, which I believe is Anthony. However Victor MacLaglen is always referred to as MacChesney or Mac for short. Even Sam Jaffe gets two names as Gunga Din.

So how come? I guess it will remain one of the great mysteries of the Silver Screen. While the movie was inspired by the Rudyard Kipling work called Gunga Din, it was not a book, but rather a poem of moderate length. In it only Gunga Din is mentioned, and no reference is made to any of the three sergeants.