It does appear to be an old tire size for a 7 inch (or 7.5 inch) wide tire on a 10 inch wheel with 10 plies. Carlisle seems to still sell tires in at least the 7.5-10 size, but I understand your desire to go with something using the more modern sizing convention.
Google quickly provided a
link for someone asking a very similar question of eTrailer.
-Rod