Although cataloged from 1913 until 1930, neither the 556 or 566 tank car was ever listed as part of a cataloged set.

Circa 1915-16 - The first two years this was actually cataloged as a No. 556, Ives changed the number to 566 in 1915, The tank was the only 4 wheel car to carry the actual number and the only proof that the catalog numbers in 1913 and 1914 were correct. Although the car shown above and below has the later type II frame, it has the early number (556) on the sides and the end (below). For the experienced Ives collector there are several indicators that this car is very early, note the shade of gray, the blackened wheels, the crisp details of the frame and the couplers are riveted to the frame. 

Side view of the above car showing 556 numbering, count the rivets and compare it to the later tank

Bottom view of above car to show rivet couplers as opposed to the later twist tab types.

Here's a perfect example of how difficult it is to date these 4 wheel cars, above car has the early type I frame that would convince most collectors to call it 1913-1915. Look closely, this car has the later number (566), the later shade of gray paint, nickel wheels and we don't have to even turn it over to see the twist tabs from the couplers sticking up through the frame. Also note the this tank has the 100,000 capacity stamp, a characteristic we believe only existed in the 1917-1919 era. The car below may look identical, but the metal used to form the tank was placed in the die the wrong way and the rivets run horizontal.