The Expert Mag is based on the Magpul EMAG. While compatible with most AR-15-based platforms, the EMAG was originally made for, and remains a signature piece in almost every SA80 model (barring the L85A1 which can only use STANAG magazines).
The Expert Mag has translucent windows on both sides of the magazine shell through which ammo can be seen. As of Update #65, the Expert Mag will visually deplete as the player fires their weapon.
Despite being shown with the attachment on the Queen's Wrath's old inventory icon, the Expert Mag itself does not come with the rifle by default and requires the player to find it via card drops before they can attach it onto the weapon.
Stats-wise, the Expert Mag and L5 Magazine are identical in performance, so the only reason to choose the the former over the latter would either be the relative ease of the former's acquisition, or down to aesthetics if both magazines are available to the player.