These are Weatherby inserts. The ones that came in the previous generation boxes that had color pictures of deer, elk, etc. based on caliber. They are one-size-fits-all inserts for all cartridges based on the .375 case, so I use them for .257 Wby, .300 Wby, .340 Wby, 7 mm Rem Mag, .300 Win, .338 Win, .375 H&H, etc. They work too for smaller rounds (e.g. .270 Win) although those rattle a bit.
I ground one insert on the belt sander to remove on side (took all but 2 minutes) because the box will "only" take 15 rows of 10 rounds.