If you have registration number, then you have a registration certificate. The model and manufacture of the boat should be listed on the registration. If there are only registration numbers but no "pink slip", then I might consider running away.
Next, depending on the year, there might be a HIN (Hull Identification Number) located on the transom in the upper right corner. Might be is the operative word.
Check the mainsail. If it is original it might have the manufactuer's logo on the sail. Might is the operative word here.
Since that is a nice photo, I am sure someone here will tell you everything you need to know about that boat.
Good luck