The "mantlet" is the front wall of the turret. This kit provides three different mantlets, some of which are obviously intended for future kits.
The kit's instructions tell you that two of the mantlets, parts A1 and A2, are suitable for these Tunisian Tigers. But this is almost certainly wrong. The mantlet corresponding to A2 was supposedly not available for these Tunisian Tigers, and every known photograph shows mantlets corresponding to part A1.