Underweight Old German Thaler (Saxe-Alt-Weimar)

Looks like a poorly cast fake.

I agree with Albert.

A specific gravity test would reveal much, but we can't do that here.
Since we are limited to pictures, here is one sample I have from many other coins.
This is an old German Thaler.
More and different pictures showing coin porosity can be provided as examples.
Use a good magnifier on yours to see if contains tiny pits, holes or blobs.
I can also show you an example where one side looks "clean" but the other side looks porous.

@albert

thanks for the detail responses and the sample picture. Looking through a loupe the irregular surface of the field becomes more apparent. The silvery area seems to be above the darker troughs in the field and while there are no blobs at all, I think the troughs I see are the pits you describe. On the reverse, no bumpiness of the silver-colored areas as with the obverse, although I find it very strange that not the entire surface is silver colored. Pockets of dull, earthy-grey are interspersed with the silver. I don't know if toning on such old coins happens this way, but my personal thoughts are that it is indeed silvering on some sort of base metal, perhaps brass? Now that I think of it, I have a 1780 1/2 Thaler of Maria Theresia and it exhibits the same sort of dull brassy color with trace silver! My god.... I think I can expect the worst now can't i?

@paulCT

thanks for your observations too. I'm not acquainted with the visual differences between corroded and better cast surfaces, since cast surfaces to my mind exhibit prominent blobs and clearly porous surfaces, which in my case aren't present as far as I can tell. So it is safe to say that in no circumstance should I expect such a weight deviation of an early german thaler?

Now I'm in need of another form of advice. How can I possibly persuade/argue with a seller who has a no-return policy to refund me? He says all coins are 100% guaranteed genuine, but as it stands now this coin very likely isn't. Will the seller have the upper hand since he is a long time dealer with only 1 negative( not on grounds of genuineness) among over 10k positive feedback?

Strangely, someone on another forum opines that the toning is nothing to be concerned about and that the trace reeding is a sign of mount removal which speaks for its authenticity, but I'm wondering what could possibly be stopping counterfeiters from doing exactly this to fool people. It wouldn't be difficult after all.

If you bought this on ebay, I understand the ebay return policy for this type of item is routinely forced on the seller regardless of the seller's stated policy. The coin is significantly underweight -- that should be a good enough explanation for you believing it is not genuine. In the future, I recommend you request an accurate weight from sellers before bidding/buying.

sg93 Ask for a refund,open a case against him stating that the coin is not genuine!If the seller accepts the return or not is irrelevant as you'll 100% get your money back,sooner or later!Ebay always backs the buyer, I know that from experience.You can contact customer service for more details if you think it's necessary.I would not rely on someone's flawless feedback because some dealers do sell fakes from time to time so you should always ask for coin's parameters before buying!Weight aside , I don't think you should purchase a coin with that obv. surface ,especially when the item is not rare at all!Coming back to your question I have to say that I'm not aware of any German thaler type that has a 2-3 grams weight variations !
I have a 1-1/2 1655 taler that's 2.5 grams underweight( 40.6 instead of 41.5- 43)and was very concerned until I had it checked by Kunker!

I have seen some really deceiving fakes of European large silver coins.
This is not one of them.

For a cast forgery your coin has certainly achieved some interesting tone. It certainly looks like a cast based on details and the fact that it's really underweight does not help, but I can't say it's a cast with a high degree of certainty just yet. Modern casts wouldn't really look like your example and I'm not convinced this is a contemporary circulating counterfeit. Can you post some better images, especially of the reverse. So you are saying this coin weights 25g? Have you tried performing an SG test? Where did you acquire this piece, and how much did you pay for it?