Prior to the Radio Act of 1912 United States amateur radio callsigns were chosen and self-assigned by the aspiring ham operator. In 1911, Hiram Percy Maxim's assumed call was SNY.
In 1912, Irving Vermilya, 1ZE, received the government's "Skill Certificate No. 1", becoming considered as the first licensed American amateur radio operator. It is noted that prior to the radio act, the airwaves were fairly wild, wooly and not especially well disciplined.
The code requirement was 5 wpm, the written exam included essay questions, diagramming transmitting and receiving equipment and questions about radio laws of the era. There is no official documentation how the USA was ultimately assigned the W and K.
In this era, an informal system of prefixes evolved. Hams used A for Australia, B for Belgium, C for Canada, etc. This single-letter system worked until Amateur Radio spread around the world and there were too many countries for the system to accommodate. Thus, in 1927, a new system took effect using two-letters with the first letter indicating the continent (E for Europe, A for Asia, N for North America, F for Africa, etc.) and the second letter indicating the country. Stations in the 48 United States used an NU or a U prefixed call known as "Intermediate Prefixes".
American amateur calls like 1AW, 6OI, 2MN, and so forth are explained by amateur stations did not qualify for international call signs. The USA was divided into nine radio districts, including Iowa in the 9th district. The 10th, or 'zero' call district began post WW-II.
Amateurs were granted calls consisting of their district number followed by letters, the first letter was from A through W, for example, 1AW.
Recognition was afforded certain land stations. The first letter in a Special Land station's callsign identified its licence class; X for Experimental, Y was Technical and Training School. Z meant Special Amateur.
The first official listing of licensed land stations appeared in the July 1, 1913 edition of Radio Stations of the United States INITIAL "SPECIAL LAND STATION" LIST wherein the very first listing was:
9YI - Ames, IA, Iowa State Coll. of Agri. & Mechanic Arts
As stations were deleted, their old callsigns were often reassigned to new stations. During World War One, all outstanding licenses expired. All stations got new ones when licensing was restored after the war. Most first licensed stations were given the same call which they had before the war. W and K prefixes were first assigned to American hams beginning on October 1, 1928.
Now, about QSL cards:
The claim is the first QSL was issued in 1919 by American Mr. C. D. Hoffmann, 8UX, but there apparently is none in existence. Also missing is support for claim of 2UV to have issued the first authentic QSL card in Europe, or the date it was used.
However, the magazine "Wireless World" published a postal card printed with the call 8ML in their issue dated May 5, 1923. The caption says, "one of the specially printed cards circulated in America by members of the ARRL for reporting the reception of experimental transmissions". The magazine advocated adoption in the United Kingdom of a similar type of card for acknowledging reports.
I make no claim to the 100% accuracy of the above; few private individual records were kept or exist today. These were pioneering times, little thought was given to the historical value to later generations.