Brass fittings numbering

Many moons ago I worked in a hardware store selling, among other things, brass fittings for use with copper piping. All of which went by numbers such as 310, 311, 312, 316, 348, 372, etc. (IIRC other plumbing fitting types also had similar numbers in different ranges..)

During the time that I worked in said establishment I ended up having the numbers and the related fitting type, plus their location on our wall bins of fittings burned into my memory. But I never really passed much heed on the numbers themselves.

For some reason this fact popped into my head this afternoon and had me idly wondering, where do the numbers come from? Are they part of some standard? Where would one find a complete list of them and their related parts?

Ta, W.