Hi Pointer and welcome to the forum!!! Great folks here with lots of knowledge and helpful info. I'm sure you'll enjoy it.
The American Arms Derby is a true sidelock gun (5 pin) there were two makers, Francisco Serriugarte and Zabala Hermanos. The Derby was available in both a coin or case color finish and also with SNST or DT. I have a Derby made by Serriugarte in coin finish and ST. It has been a good gun, but I did have a problem with the ST, which is now resolved. The fit and finish on mine is very good, not great but good. You can tell the difference between the S.F. and Z.H. by the engraving pattern and the F.S. is stocked to the fences.
As to value, it all depends on the maker as well as condition and also if the gun is a two bbl set. Values I've seen run from $600 to $2100.
Hope this helps!!
Again, welcome to the forum!!
Greg
