If you were a youngster who collected coins, you learned to scan dates for the tiny mint letters that denoted the origin of the coin.

The tiny “D” represented the Denver branch of the US Mint.

Private assayers and smelters had served Colorado miners and adventurers for many years before the US Congress authorized a mint in Denver.

Since the early 19th century, alluvial nuggets were captured from Colorado waterways and brought to the private assayers and foundries to be made into coins or bars.

(Shaft mining or Pit mining began later)

In 1862, the US Congress authorized a branch of the US Mint in Denver, but the Mint did not produce coins; it only performed assay functions.

(Due to the Civil War, the shipping of coins from Denver was considered too risky.)

In 1897, the Congress authorized the production of coins at Denver.

The US Treasury Department constructed a beautiful building for the operation in Denver.

https://en.wikipedia.org/wiki/Denver_Mint

The huge machinery required for pressing coins was first shipped to St. Louis for demonstrations at the World’s Fair of 1904.

So, in 1906, the Denver Mint first produced coins with the tiny subscript “D”.

This postcard illustration of the Denver Mint was printed soon after coin production began at Denver.

Published by Barkalow Brothers Company of Denver, this postcard was not mailed.

It may have been collected by one of the thousands of visitors who took tours of the Mint.