There are exempted from the computation of the amount of the sales tax the rentals payable under a lease of tangible personal property for any period of time for which the lessor is obligated to lease the property for an amount fixed by the lease prior to August 1, 1965, and the lessor did not elect under Sections 6094 or 6244 to pay use tax measured by the amount of the rental charge. The lessor shall be deemed not to be obligated for any period of time for which he has the unconditional right to terminate the lease upon notice, whether or not the right is exercised.