pictures of European Harmandia galls, which are found on Populus leaves. I did not open them up, so I don't know who was inside.

I do see the resemblance to some on this page. Gagne states that Harmandia "is evidently a large and poorly known genus restricted to [poplars]" but only two Nearctic species have been described from particular galls. It's possible that Ron Russo's book on western galls, which is newer, has these galls in it, but probably not, based on Joyce Gross's comment under this image:

These would definitely be worth trying to rear if you find them again. I think it's quite possible that no one knows what species is responsible for them.

the tree is on a friend's property and therefore easily reached next season. Shall I move this image for now, until I can get a better idea?

moving to "unidentified galls" would be safest, although I'm leaning towards your proposed ID. We know the first one I linked to is an aphid gall because Lynette cut one open, but those seem more irregular and wrinkly. Then again, a couple of the ones in your image are starting to get similarly wrinkly. It would be nice to know if those aphid galls start out red and smooth like this. I know of a journal article that might help answer that... I'll take a look at it eventually.