John,
The CERTUS’ “big game” rifles in the Nitro Express calibres are magazine rifles.
There were options as to magazine capacity as I have seen one on a video with a blind mag that held 2 down. Mine has a flush floorplate mag (seen in pictures) that is claimed to hold 3 down but I have not had it in my hands to confirm this yet. I also have seen catalog drawings and photos of a removable “drop mag” option that hangs maybe 5/8” below the stock and that one seems more likely to be the 3 down version.

EDITED: Somewhere, maybe in old references, I have seen mention of early CERTUS rifles that were single shots.
My C&H catalog mentions that in about 1899-1900 they did a redesign/upgrade to the CERTUS, thus perhaps to accommodate the Cordite Express cartridges and magazine?

Interesting articles on the CERTUS here:
https://www.countryclubuk.com/ci/ci76_rifles.pdf
and here:
https://revivaler.com/cogswell-harrison-certus-big-game-rifle/
- Mike