Any excise licence granted by the Government of Southern Ireland shall, without payment of further duty, be available in Northern Ireland unless and until the Parliament of Northern Ireland otherwise determines, and any excise licence granted by the Government of Northern Ireland shall, without payment of further duty, be available in Southern Ireland unless and until the Parliament of Southern Ireland otherwise deter-mines :
Provided that, if the rate of duty in respect of any licence is higher in one such part of Ireland than in the other, any such licence granted in the part in which the lower duty is charged shall not be available in the other part until the difference has been paid in that other part.