lettered edge coins: how do the letters get there?

1907Quarter

is the how did a circular die squeeze down or implode its letter impression on the rim of the coin?

RGL

From the PCGS glossary:

Lettered edge: A coin edge that displays an inscription or other design elements, rather than being reeded or plain. The lettering can be either incuse (recessed below the surface) or raised. Incuse lettering is applied before a coin is struck; the Mint did this with a device called the Castaing machine. Raised lettering is found on coins struck with segmented collars; the lettering is raised during the minting process, and when the coin is ejected from the dies, the collar "falls" apart, preventing the lettering from being sheared away

1907Quarter

cool!