Tom, I was tempted to use a number more aligned with the AF S-gauge line (and your suggestion of 24509 is a good one), but the trailer is printed with the same 33509 number used on the HO-scale car.There is a nice article on myflyertrains.net documenting the construction of this WM/Riss TOFC. I recommend the S gaugers here take a look at the article if you have not yet seen it. I would have struggled with the decision of what number to use on the car. Since it is S gauge I would be tempted to change the "33" to "24" making it a one of one 24509. The number 24509 was not used by Gilbert so it is available.