That style of bolt handle is certainly uncommon and may be exclusive to Rigby.

Karamojo Bell's .275 had the same bolt handle.
We know this because that very rifle was bought by Robert Ruark in the 1950s and later presented to his godson,
professional hunter Harry Selby.
Selby's daughter Gail famously killed a bull elephant with it in 1973 on the open savanna using 175gr solids.
Bell would have been stoked!

Did I mention I like Rigbys...?