This is the Magpul PMAG AK/AKM Maglink Coupler. Revitalizing American manufacturing, this USA-made PMAG AK Maglink is a top-of-the-line coupler:

The AK/AKM MagLink is a coupler for PMAG AK magazines designed for Kalashnikov-pattern rifles in 7.62x39mm which allows the user to attach two magazines together for more efficient speed reloads, and to keep additional ammunition accessible on the weapon at all times. The AK MagLink features a two-piece bolt on design and durable, lightweight reinforced polymer construction, which resists operational abuse. Wide-set, two-piece design resists flexing and integral grooves interface with PMAG ribbing to eliminate slippage during transport or use.