Unit designators are based on the regimental system, not the brigade system. Infantry regiments got a certain 'set' of numbers, Armor another, Artillery yet another on down the line. I got this from a staff school I took many years ago, and wish now I kept that course work. I too have not found good documentation on the web regarding this.
Unit patches are division level, although separate brigades and battalions may also have distinct unit patches. There are quite a few separate brigades in the Army, but not too many separate battalions.
The YO-11 is my favorite Patch
