Today I located my piano-vocal score of Anton Rubinstein's "Die Kinder der Heide" (which I happened to have bought 11 years ago in Leipzig, I believe, at a music store in the area of the Thomas-Kirche).
Here's the publishing information:
Piano-vocal score by the composer.
New edition.
Leipzig: Verlag von Bartholf Senff.
Plates 1493-1495.
No date.
Engraved by C.G. Röder.
I'm thinking this is safe to scan and upload, but I'm asking to be sure.
"A libretto, a libretto, my kingdom for a libretto!" -- Cesar Cui (letter to Stasov, Feb. 20, 1877)
I'm quite confident it is public domain worldwide - at least for now. That could change if the powers that be decide to up the copyright term to life-plus-250 years.