Williams plastic boxcars are NOT 6464 size, They are bigger than 6464, more like the size of RailKing boxcars.
Williams plastic box cars and stock cars started life as products of AMT, American Model Toys company around 1951 or so. AMT advertised them as scale, for some reason. One possibility is that they are models of AAR boxcars, which in real life are smaller than PS-1 boxcars.