This could end up as a very lengthy explanation

Basically, the official type of appliqué armour was only fitted to what are known as "dry stowage" Shermans. These correspond almost entirely, but not quite, with "small hatch" Shermans. The Tamiya kit, though, is a "large hatch" Sherman, and only two models of large-hatch Shermans had appliqué armour: large-hatch M4 composite (the type that had a welded hull mated to a cast hull front) and early large-hatch M4A2 (75 mm) tanks.
The distinction between small-hatch and large-hatch refers to the drivers' hatches. On small-hatch tanks, these were oriented fore-and-aft, and sat on structures that projected forward from the top of the glacis plate (known as "driver's hoods" to Shermanophiles). Large-hatch tanks had a revised front that had a steeper slope to it to eliminate those hoods, with the drivers' hatches being redesigned to be larger, set at an angle and lying entirely in the forward roof plate.
In the field, crews of large-hatch tanks might have added extra armour plate, but this was fairly rare. In Europe, it was commonly done only on tanks with 76 mm guns (yours has a 75 mm) and only in certain units. In the Pacific, additional armour more usually took the form of wooden planks and concrete (plus sandbags on the roof), though extra steel plates weren't entirely unknown.
I wouldn't use steel cable myself, I found it far too stubborn

Copper wire works much better (twist some yourself from strands from an electrical cable) as it will hold its shape, but I generally prefer nylon cord.