What a great old photo! :)
Wish I knew the make of the cars. One tag is from Reno County (6) and the other from Harvey County (28.) Kansas used the size of the counties to designate the first number on the tags. Wichita (Sedgwick) Countywas 2. I'm not sure if "size" meant square miles or population.
Good old Google says the numbers were based on population size of the counties based on the 1920 census. In 1951 they changed to a letter combination for the counties: RN for Reno.