[SIGCIS-Members] History of Rome. How did they calculate?

[herbert.bruderer at bluewin.ch]

How did the Romans calculate?
https://cacm.acm.org/blogs/blog-cacm/234881-how-did-the-romans-calculate/fulltext 
Hi all:
Does anyone know how the Romans calculated with the Roman numeral system?
What is the meaning of the right-most slot of the Roman hand abacus?
Do you have Roman hand abaci (original devices or replicas) in your collection? 
Thank you very much for your help.
Best wishes,
Herbert Bruderer
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