These pics are of a double Harbeck trailer. At the time it belonged to Lin, but now resides in New York with the Gorbolds. I was about to construct an aluminum version of the dolly (see elsewhere) and this was to give me a perspective on the balance points Harbeck worked out.
The double accomodates the bunk or the strap dolly. The forward upper bunk is positioned to take the load approximately under the V bulkhead.
The newer strap dolly did fit onto the trailer, however the pin holes anchoring the trolly did not line up , hence the strap (orange) as a quick fix. Notice how narrow the entire rig is.
As much as they thought out the upper structure, the cross support bar was a half inch too close and would rub the lower boat. Not enough to scrape the boat, but the boat cover could be damaged if padding was not positioned properly.
So these plugs are all the rage in Europe, but try to find an adapter for this in the US. (Hint, try Volvo)