Alas, we meet again! I saw this very crossbow while in Hermitage this past Summer. Our bus tour of the city allowed for only 2 hours in this amazing museum.
The room it's displayed in is quite breathtaking, and contains European weapons truly sans pareil !
Correct me if I am wrong, as this is not my area of expertise or interest, but judging by the size of the slot for the projectiles, this piece shot either stones or lead balls?