Hi Vann.Interesting question. I'm trying to figure out where you're coming from with this.
I don't have an RS so I can't answer the first part of your question. I suspect the second part indicates you have the tailstock-end leg for a different size lathe. The RS came in 6", 8" and 10" centres. All used the same headstock with the bed sitting progressively 2" lower for each size. I'm guessing the 6" had the leg 2" longer than the 8" RS, and 4" longer than the 10" RS etc. Or maybe the bed is 2" deeper/shallower?