First post so please forgive any errors. This may be American Indian rather than Mexican. There is not much of a mark so I know this is a long shot. One mark I cannot make out the other mark is likely "G.V." however it could be "GN" or even "GM". My hope is someone will recognize the unique style of the links. Each link is an open ended but interlocked "S" shape. It shows much wear, has no clasp, and while being about six links short of a normal size weighs a hefty 96g.
https://photos.buckeye-express.com/pictures.cfm/29876
https://photos.buckeye-express.com/pictures.cfm/29877