They exchanged flutes one night, and then Hiromasa never saw the person again.
Somewhere along the way, it passed from the Imperial treasury to the house of Date. Namazue Rokudayu was someone with skill enough to coax the right sound from it.
Along with other noble clans who lost their status when the peerage was abolished, the Date sold many, many treasures to raise funds. This included Oni-ichimonji.
I lost the news item that said this, but about five years ago I found out that Oni-ichimonji had been returned to a museum in Sendai in 1999.
I can't find anything newer, but it pinpoints the flute to within 19 years.