Regulations called for one inch letters but these are five-eighths. Letters of this type are sometimes referred to as "Zouave" type letters since the fancy style was favored by Zouaves. However, they were used by all categories of troops. Letters of this type have been recovered from Civil War sites. The orignal examples picture above were Civil War surplus.
A typical Civil War infantry regiment had ten companies lettered A-K and omitting the letter "J" which was felt to be too easily confused with "I." Cavalry regiments and an occasional large infantry regiment had extra companies "L" and "M." Other letters were worn on headgear as part of unit nicknames or state abbreviations.