My brother in-law found this on craigslist. What kind of drive is this and are they reliable?

Can't remember their name but my dad had one on a 19' Connelly ski race boat back in the early 80's. He was running a 750-800hp blown motor and said the bits on the inside of the drive kept trying to get out! Not sure what they where rated to though?

Just checked for you. It's a "stern power" drive. Rated to approx 600hp. Very old school!