It's British, not US. As to what it fits, I'm not sure! It's not for 5.5" or 4.5" and the US 155mm gun and 8" howitzer used the US covers in British service. 3.7"? 7.2" How?
That 'Sherman muzzle cover' is no such thing, it's just a mop cover for a pull through for 25 pdr.