Referring to the "D" in "UNITED", the first "S" in "STATES", the second "A" in "AMERICA", and the S" in "CENTS." Is this the work of a grease-filled die, or something else? I know the Mint had all sorts of issues with the dies shattering on these nickels, so could this be something related to that? The obverse of this coin clearly shows a shattered obverse die.
Missing portions of a coin are likely caused by filled parts of the die - if it was shattered dies or broken dies then it would be a raised portion on the coin That is cool though!!
The cause of the broken letters is broken letters on the working hub. Quite common on shield nickels. Note that this coin was minted after 1868. Broken letters on 1868 shield nickels are due to broken letters on the master hub. Sometime in 1869 the mint changed their shield nickel manufacturing process from master hub/working die to master hub/master die/working hub/working die.