It was not uncommon for pre-war militia and some selected early war units to make use of "hat brass" such as regimental number and/or company letter designation. Thereafter it was quite a rarity less a few known surviving examples that were soldier homemade. But these were rare exceptions than a common habit. They were typically issued to new Federal units... Confederates didnt bother.

We frequently will see some confederate reenactors with hoards of Federal issued hat brass, insignias, cords, and dead animal parts adorned all over their headgear, that one might thinks looks cool, but lacks historical evidence. If in doubt, do without. Less is More...

Frederick