How to demonstrate permission for copyrighted works

[rpatters1]

I would like to create a page and upload a scan (under performance limited license) for a score by the now deceased composer Ellsworth Milburn. His publisher was MMB, but when they disbanded they returned all rights to his estate. As far as I know, he has no descendants and all rights (other than a couple of pieces published elsewhere) have devolved on to his widow. I was able to contact his widow and discuss this with her. She thought putting his material on IMSLP was a great idea. Indeed, she sent me a paper copy of the score to scan. However, in recent months she has not responded to contact attempts from me by phone or email. (I think she may have health issues of her own, but I'm not sure.)

Anyway, my question is, what do I need in order to demonstrate I have permission to upload?

[Carolus]

I remember him from my MMB days and working on getting Korean engravers to do his string quartet, which employed some unconventional notation. I was sad to read that he passed away several years ago. Ellsworth was a fine composer to work with as a junior editor - always kind and patient. At any rate, just have his widow send an email with her blessing to copyright@imslp.org.

[rpatters1]

I did not know Ellsworth well myself, though I had the opportunity to visit with him on a couple of occasions. As I mentioned in the OP, his widow has not responded to my emails in recent months, so I don't know whether this will work. But I have sent the request.