That looks very similar to the ones that Hunter used on their H26. I replaced mine once
Yes ... rivets hold it in place. I purchased a "mast step" and it came with both parts, the base and the insert that goes in the mast. So I think that is a good way to start a search. Who made your mast? Taking a guess I would start looking at Dwyer
www.dwyermast.com
And see if you can find a profile that matches the dimensions of your mast. Then order the foot for that mast.
Calipers will make the measuring process much easier.