Offered for sale is this 1920s Vintage Felt and Tarrant Comptometer, J Model.
A pretty cool recent estate find.  Researching the machine, it appears to be from around 1920.  

Description from web search:
The Comptometer was the first successful key driven adding and calculating machine.  "Key driven" means that just pressing the keys adds the numbers entered to the total - no other action is required - so it is very quick for adding long lists of numbers.  The basic function of the Comptometer is addition.  There is a column of keys (in general 1-9) for each decade.  When a key is pressed, that number is added to that decade, with carry to the next higher decade, if applicable.  Pulling the lever forwards clears the total to zero.

Comptometers were very fast in operation when adding up lists, such as required in accounting.  Operators were specially trained to make use of the full keyboard and enter each number by pressing all the digits in one go using all fingers, as necessary, at once.  In other words all the digits were entered in parallel, the mechanism being able to cope with this.
In contrast a modern electronic calculator only has 10 digit keys so the digits of each number have to be entered one at a time, serially, which is slower.

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