Joseph Singer was a Los Angeles gunsmith best known for his triggers in schuetzen rifles. He apprenticed at Lefever Arms Co. I understand, as Daryl said, that he brought Gus Pachmayer to the U.S. and housed him until he was financially established.
Singer also "put up" Harry Pope after the earthquake destroyed his shop until others helped Pope set up again on the East Coast (Jersey City).
I looked at the double you are talking about at Las Vegas - it certainly looked like a great gun. I have a 12 marked Singer; my guess is he was just the retailer. The seller on the gun you are interested in knows a great deal about Singer - you should ask about that as well as details on the gun.
