Any interest which exists on June 7, 1973 but which has not then become indefeasibly vested, or the taker of which has not then become finally ascertained, or of the existence of the transfer of which the beneficiary lacks knowledge, may be disclaimed after June 7, 1973 in the manner provided in RCW 11.86.031. However, for the purposes of RCW 11.86.031(2), the date on which the beneficiary first knows of the existence of the transfer shall be deemed to be the date of the transfer.