there is no square drive in end of speedo
There were two Stewart 490 speedometer heads made. The 1926 model had no male screw-mount for the cable. The cable mounts with a small, off-set screw-socket. The 1927 had a more conventional screw-mount for a threaded collar on the cable. Which one do you have? A photo would help a great deal.
this is a 1926 the cable slips inside the head. the speedo has a round hole is there a square insert. or is the hole striped.
There should be a square socket for the cable-end. Is the square key on the end of the cable fully inserted? I can't imagine the cable having enough force to strip out the hole all the way down, but if the cable is slightly too short, like 1/4 inch, it can slip in the end of the socket and eventually make it round.
To make mine work right, I had to insert a 1/2-inch shim (a piece of steel rod) into the drive-head socket to make the cable engage the square socket in the speedometer head correctly and make it turn. When it turns, the cable has a natural tendency to run away from the speedometer and hit the stop in the drive-head.