Virtually certain it was made by them, "Riverside tool co" was registered as a trademark in New York in 1899. JM has a pair marked with both R.T Co, Trademark, and the anvil mark. The anvil mark fairly consistantly appears on hand saws fully marked Riverside Tool co.
All that gets you is an oldest possible' date, there seems to be virtually nothing online about the company (except rather dubious speculation, no, it's not a montgomery wards product ; P)
There was also apparently a Company with the same name in Boston Mass, also making hand saws...