See Early Princess Cars for frame progression

Circa 1904-05 - The examples above and below are the earliest of the No. 126 Caboose we've been able to locate. they have the 2nd series frame.

Another early 126, although it would appear to have a door from a much later stock car.

Circa 1905-1908 - The No. 126 caboose above and two below have the third series frame with the red/black steps, note the litho cupola with the number between the windows and the arch top to the window lithograph. All three of these No. 126s are blue (note the coloring around the doors. 

The example above uses a Princess door. Doors are easily interchanged, but Ives used just about everything for caboose doors.

Both the 126 above and below have the 4th series frame, but the example above has the number lithographed on the cupola and has arch top windows. The example below has a plain cupola and no arch windows.

4th series frame with painted roof and cupola roof. Late windows - no number on cupola.

Circa 1909 - The caboose pictured above and below would be the latest of all the 126s, note the 5th series frame  and square windows. Difficult to see from these pictures but around 1908 when the litho changed to square windows, there was also printing on one end reading "The Ives Miniature Railway Lines". This late caboose is yellow with gold coloring around the door. Also note use of another different 'non-caboose' door.