I think I’ve found the answer. If you look at the earlier prototypes, they have tanks/cylinders under the main body at each end on both sides which the earlier models have as well. In the modified prototype 221 and 222, those on the right of each side are missing as they were presumably redundant. So they’ve been omitted on the production model LGB 24450 too. They weren’t omitted on the preproduction models. I think that, in the model, the steps are secured by a tab underneath the tank/cylinder do they have been omitted from the model too. Does this make sense?