The fact it is a yawl would suggest it is probably an older construction. In the late 60s and 70s, ferro boats being built at home or in people's back yards were everywhere. If you cannot verify the name of the builder I would not be too worried.
Ferro cement is a good medium for boat building and really the only thing you need to be concerned about is the quality of the plastering and the depth of the armature beneath the surface.
Because of the number of ferro boats built by people with more enthusiasm than talent, there are some absolute terrors out there. There are also an equal number of very fine, strong boats. My advice would be to have the vessel surveyed by a professional with a history of assessing ferro cement craft.
Many areas are crucial for example: Is the deck hull join ferro and does the ferro extend across the deck, or is there a mere butt joint between the hull and plywood (are they marine ply or wholly unsuitable structural or exterior ply) decks? Are there any twitchings showing through the hull either inside or out, and how are the bulkheads attached to the hull. It is vital to have the boat surveyed on the hard and preferable to have it looked at before any nasty bits can be covered by a fresh and lovely coat of anti fouling paint.
The final test can be performed by specialists with a mobile x-ray. They can determine the thickness of the hull, whether that thickness is uniform and crucially, whether the hull was plastered up in one 'pour' or not.
If you perform an in-depth search of Cruiser Log's archives, you will find much on ferro boats.
There is no substitute for a top class out-of-the-water-survey.
Finally, I would ask you to consider the general appearance of the boat as she sits. Good boats will look like good boats. S**theaps will look liks s**theaps.
Best of luck and please, keep us posted.
"if at first you don't succeed....Redefine success"!