Abendschon would probably translate "pretty evening." An Obenchain friend of mine said, also, that it came from this. But that does not make sense, especially for the practical Germans. But abend does mean evening, and schon does mean pretty (when spelled with the two dots over the o, i.e., an "umlaut o.") Otherwise, the world schon means "already."
Oben means above. But that would not work. So I guess it is abend, as you say.
By the way, if you have enough interest to research this Obenchain forum, and enough savvy to know about "googling" the word Abendschon, seems like you'd do it too. Did you? I did, and got nowhere. So, I guess we are stuck withthis mystery awhile. I am going to see if there is an Abendschon forum. Tata, and thanks.