Major Berg, who you might remember from the Hellcat article, worked on the GMC M10 as well. The applique armour, the mounts for which are a distinguishing mark of this vehicle, were his idea.
Due to the difference in weight between the Sherman and the M10, 14 mm thick applique armour could be attached to the hull and turret without harming the performance of the vehicle. The total weight of the armour was 2031 lbs. However, the additional armour would only be mounted on a small percentage of vehicles serving in a special role, with the armour removed when the vehicle returned to its normal role.
Additional protection of this type was also considered for the M7.
Three types of this shielding (right against the armour, 1 inch, and 10 inches away) were tested at Aberdeen against M74 37 mm AP and M62 3 inch APC. The protection against the 37 mm shell did not differ depending on the distance, but protection against the 3 inch shell was better then the extra plate was right up against the armour. Interestingly enough, the protection with spaced armour was not any worse than a piece of armour of equivalent thickness.