The basis of rights to buy stock which are excluded from gross income under section 305(a), shall be zero if the fair market value of such rights on the date of distribution is less than 15 percent of the fair market value of the old stock on that date, unless the shareholder elects to allocate part of the basis of the old stock to the rights as provided in paragraph (a) of §1.307-1. The election shall be made by a shareholder with respect to all the rights received by him in a particular distribution in respect of all the stock of the same class owned by him in the issuing corporation at the time of such distribution. Such election to allocate basis to rights shall be in the form of a statement attached to the shareholder's return for the year in which the rights are received. This election, once made, shall be irrevocable with respect to the rights for which the election was made. Any shareholder making such an election shall retain a copy of the election and of the tax return with which it was filed, in order to substantiate the use of an allocated basis upon a subsequent disposition of the stock acquired by exercise.