Circa 1923-25. In 1923 IVES added an observation car to this set and for whatever reason changed the numbers from 60s to 70. Except for the 73 Observation IVES listed both 60 and 70 series cars in their 1923 catalog. The cars are known to come with either green or red roofs. 

Here's the version with the number 60 circa 1926 - although this one has a red roof it is o/w identical to the No. 70 on top

Circa 1923-24. This 71 Chair car is the variation with the green roof. Besides the observation the big difference in these cars was the lack of rivets. They probably exist but I've never seen a red 61 Chair Car without rivet detail. These cars sold in three and four car sets, usually the three car sets did not have the No 71 car included. In 1925, one set headed by a 3254 came with lights in the cars, this set did not include the No 71, but it was available separately.

Again the No. 61 version of this car, circa 1926

Normally these cars are red just like the rest of the series, but this one is Maroon. It was originally lithographed for The Wanamaker Railway Lines in 1924, but IVES wasted nothing. They painted over the Wanamaker markings and rubber stamped this one THE IVES RAILWAY LINES. This car is also known to come in red with the number 62 on it. (See Section on SPECIALS for Wanamaker Trains)

This one is lettered as a No. 62. All the 70 series cars can be found with 60 series numbers on them. If there's any other differences we're not aware of it. 

This would be the No. 72 Drawing room car probably circa 1923-24 with it's green roof, below with a red roof

The first 6" observation car, it was really nothing more than a No 72 body cut off at the door and an observation platform added.  Above is the green roof variation, below is the red roof variation.

Above is the common 73 observation from this period, below is the same car numbered 68