Whatever was installed was builder grade, just a weird way of doing it with no easy way to change it up at a later date. The bolts line up, so I'm gonna give it a go.
Btw I called a local plumber and they knew what it was and said it was a "box", and that to change it up they'd charge $300... The fernco seems like a screaming deal.
Good work! As others have mentioned, use some dish soap to help seat it. makes the drop in so much easier. slips right in.