There are many aspects to dating Mexican jewellery but the hallmarks fall roughly into the following decades (this is a rough guide only as plenty of artisans mark their jewellery outside of these conventions):
Simple hallmarks SILVER, STERLING, PLATA + MEXICO/TAXCO are preceding the eagle system, introduced in 1948.
An EAGLE essay mark was introduced in 1948 and discontinued in 1980.
The number and letter registration system eg TH-10 was introduced in 1979/1980. Essentially the first letter identifies the location the piece was created, eg T = Taxco, M = Mexico City, and the second letters is an initial of the artists name and the number is the number of individuals registered with that combination. So TH-10 would be Taxco artist, Hernandez and they would have been the 10th artisan to register that combination.