I went through Sana'a in the 70's and whilst there picked up a some Maria Theresa Thalers. ISTR that at the time Thalers were legal tender in Yemen and in the market enterprising stall-holders had silver-soldered bits of cut off coiled-spring curtain track (as used to hang curtains on kitchen doors) to the top of the Thaler as a loop through which to thread a neck chain.
The one shown in my photo makes a chunky necklace which my wife wears occasionally.
Thalers bearing the date 1780 are not worth much, too many were made including many made by the London Mint.
FWIW the Maria Theresa thaler bearing the date of 1780 is a "protected coin" for the purposes of Part II of the UK's Forgery and Counterfeiting Act 1981. Quite interesting in view of the discussion above about counterfeit goods as The Act makes counterfeiting legal as long as it's done by the government!