Sorry xausa, I maintain what I wrote.
To have examined several magnum systems manufactured before 1912, only versions for rimmed cartridges [400/.350 Rigby] have a slanted magazine.
The first magnum actions were made with stepped receiver front ring but the majority had a standard deep straight magazine.
You'll find such rifles pictured in Jon Speed's two remarkable books.
Of course, this does not detract from the elegance of your beautiful rifle.

DORLEAC
www.dorleac-dorleac.com