I think they are Rat mags made by Z71 Rat or soething like that. I'm interested myself since I have my 1st .308 on the way, a model11. Might try the search function for Ratmags.
Z71rat makes "Ratmags" that I believe are a conversion with another mag by putting the two together - I am sure he will post here as soon as he sees it.
I make a 10 round (308 family) mag like this (Aluminum box bottom, custom AR style spring, and aluminum follower):
I have only a couple left at the moment, more will be coming soon. Christmas put me behind on gun work and I am working at catching up.
I also make 5 round WSM mags (but again I need to get more made up) and extended releases plus a few other products when I'm not building rifles.