It's easy to differentiate between Zwerver shown above and Zwerver II (design #1142), even though they look very similar and both were built for the same owner. The original Zwerver has a fractional rig and the Zwerver (II) seen sailing and racing around New England for the past couple of years has a masthead rig. There are more differences: both are of "composite" construction but this design has alternating frames of oak and galvanized frames whereas Zwerver II has all galvanized framing.
This boat was built by the DeVries-Lentsch Shipyard in Holland. She was launched in 1939. (Zwerver II was launched post-war in 1956.)
The plans are in storage at Mystic Seaport and we don't have much on hand. The following is a scan of a microfiche film we have in our archives here. It's the construction plan and it does give a good impression of the boat.
Here is a current image, now named Khay Yam.