Nice catch!

According to the Maemo Codenames wiki page, RM-690 has been known about, but only from MeeGo source code mentions and it seems that no one has reportedly seen it in the wild, so might have a pretty unique piece in your hands. You could add some info to the wiki article, as I don't think RM-690 was expected to be a N950 model (neighboring devices are RM-683, a N8 prototype, and RM-696, or the N9).

Maybe it was some small step towards Lauta (RM-742) after the N950 release? Or was it just supposed to fix some faults of the N950 in a second revision?