I just did some research on a bit of a unique magazine that came with this rifle. Not sure how it ended up in my rifle as it is from back when I was just a kid.

I had never seen one with this floor plate or follower, so looked them up. Kind of interesting.

As far as I can tell, Simmons Precision had a .Gov contract and only produced these 20 round magazines between '67-'71. If anyone knows different, chime in, I am certainly not an expert.