Quote Originally Posted by Gottimhimmel View Post
But this use of "по + case unknown" is thoroughly confusing me
The case is prepositional. По + prepositional case means upon ‹some event›. This case government of the preposition по is almost obsolete. In colloquial speech, it is supplanted now by по + diative (e.g. «по возвращению в деревню»), but the latter is not normative for now.