Info about firing the main gun from Hunnicutt's definitive book on the Sherman:
"In the original design, the elevation arm was clamped directly to the gun barrel by a split ring and the cannon could not be fired until the launcher was jettisoned. This feature, combined with jettisoning difficulties, made the tank mounted launchers extremely unpopular with the troops. Field modifications attempted to correct this condition by removing the elevation arm from the cannon barrel and attaching it to the top of the gun shield. This permitted the cannon to be fired with the launcher in place, but the smaller movement of the gun mount limited the range of elevation.
"On the T34E1 launcher, the elevation arm was attached to extensions from the gun shield thus retaining the full elevation range and allowing the cannon to be fired without jettisoning the launcher."
