Prior to the 1970's, the Bach 1st valve slide was configured with a brace in the center of the two outer tubes. It appears that between 1970-71, the change was made over to the current "reversed" tube on top that we are all familiar with. This change occurred between the serial numbers of 50,500 and 60,300.
Below, we can see this illustrated. The top right image shows the 1928 Strad; bottom left, 1965; and bottom right, the current configuration which started between 1970 and 1971. Click on image to enlarge.