The Heart stamp is a proof mark only found on the FSK15.
There are five serial numbers on each model 1908.
Receiver and Barrel
If the rifle was converted to the FSK15, three additional serial numbers were added. One to the knurled Bolt lock (keeps gun in auto mode). one to the 30 round magazine release ( the new release blocked the view under the trigger guard) and one located at the front of the gas tube.