Someone might know more than me - sure looks like it started as a common "No. 1", so that is probably the magazine that you want for it - a No. 1 magazine, not a No. 4 magazine. Easiest tell on the 10 rounders is that the side grooves on a No. 1 mag go all the way to the bottom edge - the grooves stop before the bottom on a No. 4 mag. Notches and locking lugs a bit different on rear spine of the magazines as well. Over the years, people have made one work with the other by filing, etc.
Is a bit unusual to me to see a sporter with that much work done to it, (checkering, multiple leaf rear sight, patches into the stock inlets for original rear sight protectors, etc.) and the magazine cut-off still there. Shows how few of them that I see and handle!!!