National Stock Number

A few years ago, Aircraft Bolts and Nuts used to be ultra expensive which forced the engineers to use low quality ones to ensure that they are in-budget. Fasteners were also quite expensive and hard to find, the purchase normally involved visiting the warehouses, or perform extensive searching to look for them. But today, the things have changed; manufacturers of Aircrafts have the ability buy these bolts and fasteners online without having to travel to the warehouses manually.

http://efastfacts.com/blog/aircraft-bolts-and-fasteners-why-you-shouldnt-neglect-them.html