Maria Theresa Thaler signature S.F

Hi
I have a Maria Theresa Thaler which I was hoping to be able to date however the signature below the bust on the obverse is S.F without a second "." after the F any ideas anyone?

From My understanding all coins struck after 1853 had the signature S.F.

A very interesting website about Maria Theresa Thalers 1780, unfortunately nowhere the S·F obverse signature: http://www.theresia.name/en/svariants.html

Thanks that's where I had been looking for identification.
I'm beginning to think it could be a the dot on the die blocked with something as in every other way it looks like a modern post 1986 Vienna coin