Pin with no Hallmark-any idea on origin or age

I recently purchased this pin/brooch because of the lovely stamping. I am wondering if anyone has ideas about if this is Navajo work. And any idea of age? the pin mechanism doesn’t have safety catch. Is this an indication of age or just a stylistic choice. The pin weighs about 7 grams and shows as about 91% silver


this page has great info on dating clasp pins types


thanks for that information. I will study it carefully. Here is a better view of the clasp now that I know what to look for.