A interested German Catsailor from lake Constance visited the F16 Euro at lake Como, and testsailed the Viper and Falcon.
He described the differences between both cats. The Viper had a routher look, with sharper bow, the Falcon smoother lines in the hull with nice details in beam to hull connection.
Both boats have the same amount of freeboard in the hulls.
The Falcon's mast ist further back than the Viper's mast.
The weight difference of the 6 measured Vipers was around 8kg in the platform, because of the production process?
On the water the Viper and Falcon are very comparable, the extra weight of the Viper dosn't matter much.
The rudder of the Falcon felt more precise and the boat more agile.
Some minor details in lines and rigging were different.
Later in the thread is stated, that regarding to Helge Sach, there were changes in the Viper and the newer model sails better with spi downwind, and has better handling on board.

GER3 and GER6 are two more Vipers in Germany, don't know if they are Class members. Will try to contact them.

Regards, Holger