The Gaden Tangka coins do not have date on them.
Your coin is of Type E (iv) which is attributed to 1899-1907A.D. Dod-pal Mint
For more details refer the article "The Gaden Tangka of Tibet" by N. G. Rhodes (which is available at http://www.onsnumis.org/articles to download)
You can also refer to examples of various types here on zeno.ru
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