Fürstenberg
The Fürstenberg Porcelain Manufactory was founded in 1747 by Duke Carl I of Brunswick-Wolfenbüttel in the town of Fürstenberg, Lower Saxony, Germany.
Since Fürstenberg is still in business, I do not provide information about the mark on this website.
I recommend you visit the company's official website.
Link: Fürstenberg website
Old Mark
SEIT means 'Since'
You can estimate with ‘west’ Germany and just Germany.
But some have just their mark.
FYI, in Germany, West-East division dates from 1949, and reunification dates from 1990.
Teacup Trio
Pot
Mark
With underbar
With number
With dot