Actually Anna Komnena, describing "Franks" - that is Normans described roller nut crossbow in the time of first crusade.
Marc, can you post a picture, I have made few, perhaps I ll know where is problem.
We now think that they used moose antler roller nut and I have checked and there was still moose in my country at 15. century. They did also imported raw stuff from scandinavia. There are examples of roller nuts made from thinner pieces of antler, riveted together, but I think this was only a emergency solution and all I know are from time when crossbow ceased to be military weapon.
This is what I made, but its the type with a notch lock.
Jaro