There was a link to a free pattern listed on here a few weeks ago, for a before and after maternity top. It had an inset which had diagonal buttons, like the one in the photo you've got, here. The only problem, is that when I went to get it, the page or site or something won't go through. So try contacting www.canadianliving.com
which is where it was, and ask them about their free before and after knitting pattern. The other thing is to search through www.knittingpatterncentral.com
and see if there are any tops with that kind of buttoning and then you could maybe just figure it out from there?. . .
Good luck with it-- it's an adorable outfit!