Oxygen is your real problem with Silver...although other things play a part, body pH, the general environment, even the holster. Expect silver to eventually tarnish in some way unless well protected. Sterling silver (the .925 marking) is a copper alloy, with some other metals mixed in. It will tarnish, and the rich once employed people to polish their silver for a reason.
Gold tarnishing is more of an issue as it is much more resistant. There's no 24 karat there. Even 10K, 14K, or 18K marking may not mean they are "pure" 10, 14, or 18 K gold. Gold is also "Plated" or "Filled".
Gold plating is found commonly over a base metal, like sterling silver, which would make sense. Gold content must be at least 10 karat and two microns thick. Gold filled, must be must be 1/20th or 5% real gold. And there are specific markings that should come with each.
These don't have those markings but given your sourcing, Mexico, it's probably likely...there's less here than more. But, again I'm guessing. If these are actually 100 years old....it's a different conversation.
All that said, what you probably have is (from the pictures) Silver, plated over (I'm guessing, but sorry,) Copper, with Silver and Gold accents either plated or filled, and probably more likely filled than plated.
Either way, they are VERY cool. A careful cleaning and examination should be very interesting and look great. And even then, even if completely faked, they might be a very nice BBQ grip.
And, I hope I'm the most wrong guy who has ever lived. I hope these were made by Pancho Villa.