There were atleast three manufacturers of the Derby, F. Sarriugarte and Zabala Hermanos as noted above. Armas Erbi also built them---they were known as "Erbi Derby's".

In regards to quality, I do not believe you can make a blanket statement that one manufacturer was better than another at producing the Derby, but rather you have to inspect and judge each individual gun to determine its quality. I have seen junk and I have seen guns that weren't too shoddy by each of these manufacturers.