Ma'aseh Rav 175 records the following practice of the Gra regarding reading Shir HaShirirm on Pesach, Kohelet on Sukkot and Rut on Shavu'ot: the reader makes two blessings beforehand, 'al mikra megillah and shehecheyanu.
What is the source for reciting shehecheyanu? None of the earlier sources that I've seen which discuss making a blessing on these megillot make mention of shehecheyanu (Machzor Vitri, Hagahot Maimuniyot, Mordechai, Maharil, Abudarham, Levush, Bach, Magen Avraham).
In fact, the earliest source for reciting 'al mikra megillah is Soferim 14:3, which then goes on to only mention reciting shehecheyanu for Esther (14:5), strongly implying that the other megillot do not have a shehecheyanu.
So what is the Gra's source / rationale for reciting this blessing?
