This is the flush fitting 4 round .17HMR factory magazine for all Anschutz .17HMR rifles (1517-U4 012961).
This is the third version of the magazine, re-released in November 2016 with an all stainless steel magazine body.
Anschutz currently don't make a higher capacity magazine for the .17HMR so most people carry a pocketful. Though some customers have reported it is possible to grind down the follower legs slightly to take five rounds.
N.B. Anschutz used to provide this model number as universal for both .17HMR and .22WMR but more recent versions have been calibre specific. For the .22WMR version the please see the 1516-U4 model.