Tom, In looking at your box pic, Your serial# is 70-47XXX. Near all of the .257 Roberts ST's I have seen have been in the 70-57XXX-58XXX range, more or less. The Roberts appears to me to have been the first production of Round Tops; anyone know different- I would like to know.
I would surmise that yours is one of the 1st made, or a prototype, and the box stampers did not know what to call it! Your father would have had access to these kinds of rifles. By the time Ruger got to the regular run series, they had their Catalog # down and an "ST" stamp! That is my opinion anyway to explain the "RT".
Nice rifle; some one ought to jump on this!
Lee