This style of crossbow is based upon a 16th century crossbow found in the Manchester Museum. It's defining characteristic is the large block of wood coming off the bottom of the crossbow.
This stock shape can be seen in many late period illustrations from various countries, especially at images of crossbow tournaments. You can see pictures of the block being used in two different manners. Some people are anchoring their elbow against their body and resting the bottom of the block in their hand to remain steady. Others, are placing their hand vertically on the front of the block, much like a modern pistol grip.