I don't know whether there was any later change, but on my 1959 5TA, which shared the 3TA frame, the stop is actually part of the lower friction disc of the steering damper. The disc is extended into lugs either side which are bolted through a central lug on the downtube to prevent it rotating. On the left side the stop arm on the forks and the damper lug both have holes which overlap to allow a steering padlock to be used with the steering turned fully left.
I hope you can make sense of this and that it helps.
Jim.