Hi, that's a very good question.
Chris Custer who started Rotus numbered the first few cars in an odd way. Mine is the first chassis, but it was actually designated #002. The next car was designated #014, and i believe the car after that was #021. At some point they went to sequential numbers.
Dennis Hedges of Short Mountain Enterprises in Falling Waters would have the best guesstimate.
Are you looking to buy one?