I just sold one. Actually it was installed on a boat. They were made for about 2 or 3 years around 1992. They use a standard alpha upper but have a very short lower unit. Nose cone built in, very big internal parts. Mine was 1.3 ratio. I had a blown small block in front of it 635 ft/lbs and 535 HP. Ran it that way for 9 years. Never had a bit of trouble. I rarely did any wide open hole shots with it, and made sure I was on plane before putting the hammer down. 21' boat.
I heard they put them behind Mercruiser 454's also.
Cannot help you with what they are worth. I do know you can still buy any individual piece for it from Mercury. Cannot find a new complete unit anywhere.
Here you go. Pic taken about a month ago.
http://members.cox.net/infomaniac/ss.jpg