[dogscoff]

The only way to achieve what you want is to create a new entry in vehiclesize.txt - more or less identical to the first one, except for the image. Then you ought to be able use mounts to make sure that your weapon can only be used on one of the ships. (You might have to make one hull 1kt larger than the other or something, to make sure that the mount can only be used on one of the hulls.)