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The mark appears to read as AIM, the rest reads as:
STERLING
925 mystery mark R.G.?
Guadalajara, Mexico
Can you clean up then reshoot the second line in particular may give you more information. A well chewed wooden toothpick with some silver polish should work.
Locating a copy of the The Little Book of Mexican Silver trademarks and hallmarks will most likely be your best resource. I found nothing online to match AIM myself.