I believe you have the original Ruger .44 Magnum Carbine (originally called the "Deerslayer").
You can confirm that by referring to the Ruger manual for that arm:
If so, it does not have a detachable box magazine, rather a tubular magazine in the forearm.
If not, check the Ruger rifle parts booklets:
—to identify it. Perhaps if you have the correct model, you can find the correct magazine.
Regards,
Walt