Well, after receiving the helpline answer, I then phoned the IRS helpline. The person I spoke to was quite certain that I have to file form 3250, although he did try (but failed) to find a way out, given that the amount of money is so small. He actually took me most of the way through the form, although unfortunately not all the way. I then had another look at the form by myself, and found the really complicated bit at the bottom which he had not gotten to with me. So I called the helpline again, but a different person answered, and would only say that Form 3520 questions are "out of scope" for the helpline, and I should seek professional advice. I'm pretty annoyed about that.