This is the 5 round Tikka T3 extended 'medium+' magazine in 6.5 Creedmoor, also suitable for .308.
The medium plus size is as per the .308 model but with a more generous COL of 75.58mm
Manufactured from a tough black polymer it features a single stack with a leaf spring and is strippable for cleaning.
COL = 75.57mm 2.975"