1780 Maria Theresa Thaler #30c Fake?

Yes, this coin looks genuine to me. I also doubted about the measurement at first. But my scale is 0.001g resolution and can easily determine 1% precision of silver content. I have measured more than 5 times with power cycle and calibration in between. I have also measured other 3 MTTs I have at the same time and the results are the same. This coin has 5% less silver than the others.

I have a digital scale. I first weight the coin and it is 27.55g. I then put a cup of water on the scale, zero out and read the scale when the coin is hanging in the water. It shows 2.727g. SG=27.55/2.727=10.106, which is 78%. I have used this method to measure over 100 coins from 1770 to 1950. The error of silver composition is usually within +-2% for crown size coins.

I'm still curious if the OP would get different or better results, or have the same question if a better suited weighing method was used.
I call attention to a scale having capacity to zero out a cup of water. And the observed delta among the coins in question is a matter of .1 gram.
My questions about equipment make & model plus specifications went unanswered. It would help, if presented in this forum, if those detatils were known.
Specifications include details such as:
Weighing capacity
Maximun display
Minimum weighing value
Repeatability
Sensitivity drift
Linearity
Full scale acuracy
Percentage of range
Percentage of reading
And so on.
Just some considerations not answered here.
For these reasons, I would not be wondering if a coin was counterfeit based on some coins being 0.1 gram more or less than others. I'd want to better my measuring abilities.
One final obvservation is that among the counterfeits that are supposed to be silver in my collection, the difference in weight and SG are much greater than the difference presented in this topic.