It's these: https://mdttac.com/223-mdt-polymer-aics-magazine-10-rnd/
Says max COAL IS 2.44"
Capacity is 10 and then the one with the extension is +5 so 15.
This is the extension: https://www.mkmachining.com/product/carbon-fiber-nylon-sa-polymer-mdt-223-5-extension/#color
EDIT: Selling 500 for $210 shipped (500 is $251 shipped on GA). 1000 have been sold as of this update.
Tested in 2 rifles and shot well. Just didn't get around to using them in matches.
I'm asking $925 shipped.
These are $1170 shipped from GAP.