Ruger 90484 Mini-14 300 Blackout Ruger Mini-14 20 Detachable Black
<p>Constructed of black steel, this 20 round magazine is for your Ruger Mini-14 300 Blackout rifle.</p>
dackoutdoors.com
I've not used this vendor before, and there are some comments about them being slow to ship.
Apparently these are 5.56 mags with the ".300 AAC BLACKOUT" lettering applied.