I believe they are the DS-4-4-660 model as described here.
That's what I thought, too, Mark. I have the NS (#661) version, which a search shows as a DS-4-4-660. But the FlyerChief (there’s one on the left in the picture above) has 3 exhaust stacks, one of the features that distinguish an S-8 from a DS-4-4-660, which has 4 stacks in all of the photos I've seen that show their stacks. Maybe the Flyerchief is a hybrid, meant to model them both?