Cooperative and foreign corporations transacting business in this state pursuant to the provisions of the Rural Electric Cooperative Act shall pay annually, on or before July 1, to the state corporation commission [public regulation commission] a tax of ten dollars ($10.00) for each one hundred persons or fraction thereof to whom electricity is supplied within this state, which tax shall be in lieu of all other taxes except those provided in the Gross Receipts and Compensating Tax Act [N.M. Stat. Ann. Chapter 7, Article 9]; provided, however, that in the event a contract has been entered into by a rural electric cooperative and a power consumer prior to February 1, 1961 and such contract does not contain an escalator clause providing for an increase for added tax liability on the cooperative, then the sale to such power consumer shall be exempt until the expiration, extension or renewal of the contract.