Hi Brett,
our vessel is the exact same Hull and is 57 feet in length. Kong Halvorsen built two models on the same Hull a 51 and a 57 foot motor yacht. From the photo's it looks like a 57 with a cockpit the same as ours, but ours was built later and was one of a number of custom Builds brought to Australia. Ours was designed here in Australia and badged a Halvorsen.
There is a sister ship here that was lengthend from 57 to 63 or there abouts post production, and is getting around fine. obviously the Props and shafts were left in the same location but the rudders were moved to the rear, and the boat performs fine. I think you might find the hull is original length, although the cabin looks like a 51, in that is closer to the bow , if that makes sense ?
Ours is very similar to the vessel you are looking at with the cockpit, if you look in the lazarette you should be able to see the rudder posts approx 2.5 feet from the transom ?
ours performs well at sea, and I note from the sales photo's it has the Niad stabilisers installed which we love also. It is an old boat so will have all the usual things to probably work your way thru gradually replacing and up dating gear.
have a look at the photo album on our page for photo's to compare.
kind regards Chris D Liberty 2015