Circa 1917, this was the very first 19. The motor is the same as the No. 17 which accounts for need to round out the area in the bottom of the casting so the key would fit into the mechanism. Note also the second boiler band on the front and that the headlight is cast into the boiler front. 

The example above is marked 1920 on the mechanism.

Just a nice example of one dated 1922.

Circa 1924 -  Single boiler band, smooth steam chest, center headlight and die-cast drivers. Below is the same locomotive except it has a lightening bolt connecting rod.

The example shown below is missing it's drive rods and the mechanism is marked 1924, but what makes this one unusual is the gold trim around the windows and cab. This look is very similar to the 1120 electric that was part of the 1928 line

Unusual casting for a 19, similar to casting used for No. 20

This one is dated 1927

1929 only  - Rare - Ives casting on an American Flyer motor.