Derrell is correct. Miller did not purchase any cars from the C&S. They were put in his name as Settlement and payment for his work in attempting to make a agreement to operate the C&S railroad. When the ICC refused the transaction, Miller had a claim against the C&S for "damages or expenses".
The value of the surplus cars was the settlement.
Now, let look at the situation of idea that RGS bought the cars as they were on their line. I do not believe the cars were ever considered anything different than any other private campany car. They were a private car owned by Miller just like the private tank cars that were running out of Farmington or Chama at the time.
Maybe there might be a ORER (Official Railway Equipment Register) where Miller registered his private cars so that they could be used on the RGS or maybe the D&RGW. It is possible that all his car numbers are registered so that the railroad freight agencies could bill the loaded cars.