You would be correct, Iab. If the last (largest) cog is indeed threaded instead of splined -- and yes, that does make it really old -- it will be left-hand threaded. It screws onto the back side of the freewheel body, the hub side, where it comes to rest against a shoulder and so has to be left threaded in order to be not unscrewed by pedaling force. I've never seen one of these but I remember reading about them in by-then obsolete books during my "indoctrination."