Looks like it could possibly be the spare rounds holder for manual feed (eg. biathlon relay: carry 2x actual 5-round magazines, 2x 3-spare holders)
Older Anschütz biathlon mags (based on the 10-round mag body) had the ability to carry 3 rounds clipped at the end of the magazine (which was capped to 5 round feed). Previously, loose rounds were just an inelegant 'drop in front / in a tray' affair to use as required. All modern biathlon stocks have holders to accommodate each individual spare round for the athlete to pull and manually feed.