The 37 is a unique crossbow that is received as a reward for completing the side quest "Within a Rock and a Hard Place". There is also a small chance to receive it as a reward for searching Unspoiled Ruins.
That is what they appear to be, the tiny numbers etched on the stock of this crossbow. Is it the number of kills? A number in a series? No one knows, and yet, a weapon this powerful should be famous across Auriga...