I think the first letter is an R. Second could be an O. I know you are good with your metals, but I have to ask. Are you sure it is nickel? I thought nickel was a hard metal. Not so easy to hand work.
Alpaca, white brass, german silver, etc Copper with just enough white metal to make it look silverish
Ok, Alpaca If I remember correctly, alpaca was the Mexican version of german silver. It is more malleable. Also it would explain the more Indian style stamping. During the 1930s and 1940s, quite a few Americans set up jewelry shops in Mexico. I was not able to find Banning, Bonning, Ranning or Ronning in any of my resources. Sorry.
Sorry to keep questioning you. Did you test for silver? Could be American Indian, I didn't find the 3 names above. I also, did not find a name starting with Running. However several of the names are stamped in the same manner as yours. Not all Native American craftsmen have been identified or cataloged. http://www.art-amerindien.com/signature_indian_native_marks_mz.htm#r
