Circa 1913-1916 - Early version of the 3239 - the wheel axels do not line up with the axel journal boxes on the frame and there is a small 'ear lobe' (below) on the end. The example pictured here is probably the very first, might even go back to 1912, even though it was cataloged that year. It's early because of the New York Central markings, sometime around 1913 Ives stopped using road names and marked everything "THE IVES RAILWAY LINES". You can also find passenger cars marked for the New York Central

Here's an example of an early 3239 with THE IVES RAILWAY LINES lettering.

Circa 1916-1920 - The last version of the 3239 can be seen in the black example above and the gray example below. The wheel axels now line up with the frame.

The bottom of a 3239 showing the power truck.

Side view of above 3239

This single brown example has been around forever, but it's originality is questionable. The lettering appears to be a different style. 

Circa 1915-1920 - look closely at this 3239, it was made up specially for F.A.O. Schwartz and company. The ends are marked 'F.A.O.S.' where the number normally appears.