Good question. Felicia.
I have no answer re the use of "Buck" but did find some interesting reading at Wikipedia,
"A nickname is a short, clever, cute, derogatory, or otherwise substitute name for a person or thing's real name (for example, Tom is short for Thomas). As a concept, it is distinct from both pseudonym and stage name, although there may be overlap in these concepts. A nickname is sometimes considered desirable, symbolising a form of acceptance, but can often be a form of ridicule. Many artists and actors have nicknames, which in years past were called a stage name. A person's online nickname may also be known as his handle, especially within hacker culture.
A nickname may be a hypocoristic form of a person's first name. This is often a simple abbreviation of the name. For most English names the shortened form is taken from the first syllable e.g. Walt for Walter. However in many other languages it is much more common to use the last syllable of the whole name e.g. Italian Nino for Giovanni (via Giovannino, which is a diminutive form of Giovanni). Some abbreviations can use the middle of a word e.g. Liz for Elizabeth. There are a few names for which an archaic pronunciation of the full name is preserved in the short name e.g. Rick for Richard indicates that the -ch- was originally pronounced as -ck-. Some other nicknames were created by rhyming the shortened form of the full name, such as Ted or Ned for Edward (Ed), Bill for William (Will), and Bob or Nob for Robert (Rob). For those abbreviations that do not begin with the same letter as the full name, see list of short name forms. Examples:"